The whole point of Heavy Thrust is to give you something necessary to do that's outside of your normal combos. If the damage boost effect was moved into Impulse Drive, you wouldn't even need to pay attention to the buff - it'd just always be up, simply by doing what you'd normally be doing anyway. Heavy Thrust is separate from your DPS combo specifically so that you have to pay attention to your buff, and work a Heavy Thrust into your rotation when that buff starts to run low.

It's a way to add complexity to the job. A cheap way to add complexity, possibly, but complexity was its intention. It's another way to differentiate the good DRGs (who keep their dots and buffs up full time) from the mediocre ones (who let the buffs slip from time to time).

The same used to be true of Hot Shot, before it got it's 4.0 makeover.