Of course there is much more to playing an fps than just aiming. The thing is what would be harder? Getting a sniper headshot in counterstrike or a sniper headshot in halo? Granted halo can be played on the PC but the majority of users play the game on console, including myself.
Halo still has movement and map positioning but so does quake and counterstrike. One of the things that separates PC fps's compared to console fps's is just how much more precise a mouse can be.
I actually find getting headshots in Halo much more difficult than just about any PC game, because aiming with the mouse is, as you say, so much more precise. Therefore much less 'impressive' when they actually happen. Also, try playing halo 2 with a mouse; you'll get shit on by a competent player with a controller; strafe is terrible with input from WASD.
Headshots don't exist in any version of quake, either. However railing someone from across the field always DID feel pretty amazing.
So the precision of the mouse doesn't really do anything to make me want to watch those games more, the best pros in Halo don't miss sniper shots either. They are ultimately very different games, but I really don't agree with your argument that any of them are more or less competitive in certain circumstances.
I'd almost say the mouse being so precise is sometimes a detriment to some games now, as the designers are trying to make games more and more realistic in terms of damage models and realistic weapons, but then you're aiming and shooting with something that gives you laser-like precision.