Every enemy stack has a counter-stack that's best for killing it. Subs are a part of the equation. Say the enemy has 3 destroyers and 1 cruiser. That's not the time to use subs. The destroyers will destroy you. But if the enemy has 1 destroyer and 3 cruisers, then a stack of 4-5 subs will do the job.

Or just build subs 1 or more levels above the enemy's destroyers and naval bombers. They can't see them. You can go wherever you want, pick your battles, hit and run. I personally almost never build level 1 naval units for this reason. I want to wait a little and get units that are a class above whatever the enemy has. And not just on fighting power, but hiding (and seeking) power. Keeping your subs 1+ levels ahead of the enemy is a huge advantage.

It's also convenient to move navy stacks together, cruisers protecting subs from naval bombers, or cruisers and battleships protecting subs against destroyers. Else it's a waste to use subs to fight destroyers, even if you win, you suffer too much damage. Once you find something worth killing, then unleash your subs. A stack of battleships with inadequate destroyer support, for example. Your subs detach from the rest of the formation, approach the battleships and engage in battle. Battleships can't shoot their big guns while engaged. This is when you bring in your cruisers and battleships to pound the enemy while the subs keep them occupied. Kill them fast, with minimal losses.

The possibilities are endless!

In a battle with ships they will make your ships take less damage per volley. Won't themselves do damage. Best to break them off from ships and run at attackers. They might draw all the fire. Separating and rushing destroyers at attacker can draw fire away from your ranged units as well and keep his subs from rushing you.

Offers good protection for subs against destroyers and navy bombers.

Personally never have them with ships. Might have a pack of them in the area of my ships but most are busy scouting. Prefer to keep them on their own not slowing down or being slowed down by ships but need to keep them as upgraded as possible. If they can't maintain stealth they are very vulnerable.

Dude, that's all backwards and confused

What's the point of rushing destroyers against a stack of capital ships?

If you do that against me, I will withdraw, fire, and keep doing that till your destroyers are dead. Now your destroyers are gone. Now my subs can take you apart, and you can't do anything. Can't even see them.

What's the point of keeping subs in a long-range fight? Just to see them take damage? That makes no sense, either. I'm not researching and building high level, stealthy subs, just to waste them as a meat shield. Those subs are going in to fight, taking the long-range guns out of the battle. THEN my guns come in behind to do their work.

Some remarks on this discussion.
- Ranged ships CAN ignore the subs they're in melee with and just defend against them, while their guns shoot elsewhere. I was very surprised when I found this out as well a few months ago, and it certainly doesn't happen often as most people don't know this and don't change the target, but it can really ruin your battle plan when they do.
- The advantage of using subs as a meat shield is that the ASW damage of ranged ships is much less than the surface damage. They absorb their proportion of enemy guns, but take much less damage than the surface ships would, so the surface ship basically last longer while the subs don't get TOO heavily battered. Not to be used when your subs are top notch, indeed, but when they're just average or below par compared to the rest of your ships, it is certainly worthwhile.
- There's a big difference between having a big stack of subs or just a few. The big stack should operate alone, picking fights where there's few enemy destroyers and NB's, while the few subs should be operated much more carefully: they're vulnerable when outnumbered even if the enemy doesn't have many ASW units. They can either stay with the main stack as meat, or use their stealth to lurk around and pick out individual reinforcements, transports, or just do general scouting.
- Rushing destroyers into a big enemy stack is USUALLY a bad idea, though there's an important exception: when you have the ranged numbers advantage but the speed disadvantage, it can be useful to sacrifice the dessies to pin down the enemy and gain time for your big ships to close the range and use their guns.

In the big rock-paper-scissors naval game, subs are probably the best unit overall; they pay for this by not being able to bombard any land units. There's also a current bug which makes them very ineffective to achieve naval dominance when the enemy is ALSO using subs, because units don't start combat when neither can "see" the other. So basically when you want to transport a lot of troops in some region, you always need something to spot enemy subs. The bug may be fixed any minute of course, and this argument will be invalid.

You are a better player then most. If you started retreating your ships I would have to chase with my ships to keep up the ranged attack. I would have to make decision whether to keep destroyers separate or whether to reunite. You would have to make decision which to target. Of course my navy bombers will be keeping any subs at bay in perfect world.

The destroyer attack works against computer players ok, and the destroyer is usually a level below my cruisers and cheaper/quicker to replace plus a damage destroyer won't slow down the fleet as much as a damaged cruiser. Against human players, such as you, it might be a big mistake.

The only human player I used the tactic against was someone who healed with gold after every volley. I had to get the destroyers engaged to keep him from healing. Plus I didn't want his two fleets to unite before my 10 subs united behind him. Every situation requires a little adjusting.

You're right, different adjustments against different kinds of players.
With gold healers, all bets are off. We need totally different tactics.

Also the Comintern cruiser is a beastly meat shield in terms of HP-per-resource-spent.
Another trick is to use high level transports, filled with cheap level 1 militia or AA/AT guns.
Adds cheap HP to a naval stack.

Adding subs to a ship stack has the added advantage of surprise against enemy subs and can cause your enemy to miscalculate. They might think their sub stack has the advantage against your ship stack, until they engage and find out you have a sub or two that shift things in your favor.

to add in on the conversation: Destroyers are the fastest naval unit if I'm correct, so using them to catch enemy stacks might be ok strategy.