To be fair, it also sort of goes along with the fact that you cannot actually target direct fire in general, and have to rely on units themselves to do so - it usually doesn't really bother me since I assume that units will sort out their own suppression fire better than micromanaging them would. Of course, that's not always true, but it usually is!
I suspect it might be tricky to put in, mostly because of AI limitations (where the AI a) isn't going to be great responding to it; b) will likely be either much worse than the player at using suppressive fire... or far too good at it (i.e. essentially cheating). it'd certainly like to have the option eventually, but given the scale at which AB usually plays (and the fact that there's always the indirect fire to do suppression with) - I actually don't think lack of manually-ordered suppressive fire has an especially big impact on his game right now.
I would like to see a bit more nuance in terms of suppression though - for example, I've already played around with that a bit in my Chechnya mod by slightly boosting the anti-armor properties of some small arms (which means they'll fire on vehicles that they have no real chance of killing, but can at least force them to button up and sap their morale). Firing small arms at armor in order to disorient it and force them to button up or evade is actually a VERY real-world tactic, as old as armored warfare itself - but at the moment in AB it's a very "all or nothing thing"...meaning that if you allow small arms to fire on armored targets, they'll engage them like they would a soft target, meaning they'll unload as though they're shooting to kill - spending lots of ammo and revealing their own position in the process. It'd be neat if units instead could be made to take very occasional, hard to detect pot-shots with intention to suppress rather than kill (especially against targets that they cannot kill themselves but could help prepare for a kill by something else).