I see nothing wrong with the "shared controls" system in principle; having more than one feature under one key is all fine, as it can sometimes be handy when you have all important things in very few buttons, right next to your movement keys for example.
Really? Because just the fact that abilities need to be prioritised, is enough to cause problems with such a system. If there's ever a context where both abilities
could be useful, you have a problem that cannot be solved with any amount of "clever" coding. Brink proved this - their choice to lump several abilities on a single button was a game-killer for me, and many other medic-favouring players. No amount of prioritising or contextual decision-making avoids the fact that the game just gets it wrong, and regularly.
Personally, I've disliked this approach every time I've encountered it. If you have two distinct requirements but their contexts of use cannot be 100% divided and exclusive, put the options on different buttons. It just makes sense. If both of these requirements are critical and their contexts of use overlap,
definitely put them on different buttons

Doing otherwise simply adds complication (prioritisation code) and problems (badly chosen priority at critical times). Seriously, I can handle having one or two extra keys to press. This doesn't spoil my fun as much as a game confusing what I'm telling it to do, and doing the wrong thing.