Every unit has its worth.Yes if you don't use certain units, don't want to use it or have no use for it then yes they would be "worthless".
Every individual player has their own play style, tactics and strategy.
AA and SPAA does make sense,every unit does make sense.
The only way to see if units does have any value (provided that it corresponds with your doctrine)is to use it and experiment with them.
Also I can't tell you that you should use them, only that if someone can't produce or use interceptors then the next best thing would be AA/SPAA for air defense.AA for defensive play, SPAA for offensive play, defending you stack against air attacks.
Hope it helps,other experienced players might give their input regarding this.

It depends on your play style. Air can be tricky for new or inactive players to use. To use planes correctly, you need a lot of screentime.

Including AA or SPAA with ground units will give them some deterrent for enemy air. Like all groups, they move at the speed of the slowest unit. If you want fast moving ground stacks that can advance out of range of your interceptor's ability to provide overwatch, SPAA can have value. AA has similar value for slower moving ground units. Interceptors in general can be tricky to rely on. Numbers matter and it's not uncommon for you to lose the advantage in the air if you don't commit to it enough. Once that happens, you're vulnerable to enemy bombers, and your own bombers become worthless. Plus if you invest in enough AA or SPAA, your own interceptors can focus more on protecting your own bombers.

It really depends on what your plan is for winning and what plan your enemy is pursuing as well.

I use them both but not in same game. Putting them in stacks with other units works great early to mid game. Don’t overlook the buffs against light and heavy armor. Usually the game action and opportunities factor into what is needed at the time.imoo

Something I can add though is that Interceptors does produce fast with no metal and food needed.
AA also produce fast with no rare material needed, higher level ordnance plants will produce even faster.Level 5 AA in a 5-6+ stack takes down a lvl 1 nuclear bomber,not sure how many interceptors.
SPAA takes longer to produce and needs no food.
It all depends on an individual's map, doctrine,resources, circumstances and what/how you want to do it.

Unfortunately, there is tricks to avoid the AA of a target stack with nuclear bombers (hint: don't target the stack itself, but something different within the blast range of the "real" target).