Is mechanized infantry + tank destroyer a good stack?

In Allied infantry isn't even bad, I'd upgrade them to lvl.2 and maybe even lvl.4 and use them to crush rebellions and as meatshields.

Also, as nerfed as they are (2 day 2 delay, practically 4 days due to Axis & Commie buffs) do medium tanks still have a place in Allied? They are more expensive than TDs as well.

All infantry is good, even if Pan Asian and Comintern infantry are better than the other two. I skip-level mine to 3 or 4 depending on how urgently I need to use them. Leveling to 2 is generally not a good idea, as it's not cost-effective.

I never build MT when playing Allied doctrine. They are slow, expensive, and the research is retarded compared to Allied TD. Axis is special, because their MT are fast and powerful enough to act as a "blitz" unit. Same role as LT, but much stronger of course. Other doctrines have MT that move at infantry speeds, so they aren't able to exploit a breakthrough. And if you're fighting on a static front line, then you should be using ranged units to do the damage while close combat units screen your shooters. This is how infantry, tank destroyers, etc. become "offensive" units.