Well currently the only way something can be accessed from the powers bar is if it is first loaded into a device slot which are very limited. It would be nice if items like hyposprays and shield recharges could be accessed directly from inventory freeing up the device slots for more important things.
There is a sense of realism with the current configuration, the kits are clothing accessories and you wouldn't realistically be changing clothing instantly in the middle of combat. A quick change keybind system would be a convenient tool but it too would not be realistic if it were something that you could use to switch items instantly while in alert status.
I think i get what your saying, you want a button you can put into your ground interface where you can press to switch kits, as opposed to going into inventory to change?
Personally I've never had a problem with changing kits, (i normally stick to one kit depending on situation, and lets be fair, pressing 'I' double clicking a kit and pressing 'I' again isnt a huge strain ^^)
Plus if there was a button to switch kits in our power trays, I would hope we still are unable to switch kits during 'red alert', could really break a lot of balance being able to switch kits at will in combat.