I would have agreed that stacking AST cards would be OP, however something has just occured to me.

You could have 30s of double strength Balance. (stacked Balance)
Or you can have 60s of regular Balance. (staggered Balance)

The net result is pretty much the same.
The latter is currently available with coordination...

So is it really that OP?
Of course with stacked you could also coordinate with other burst phases, which would increase overall DPS by more than staggered, but for the vast majority of players that wouldn't happen.