That would result in an additional icon click - the attack itself - after selecting a manoeuvre.
Well ... if you ask me, one additional click is nothing.

Especialy if that additional click gives you whole new level of functionality.

And there allways is option to hold Ctrl. :P
And, don't forget, you'll have to search for that icon, as well.
That is part of why i said i would like to see them working "simmilar to Metamagic".

We can call them "toggle buttons" if that sounds you better.

If you played Sorcerer, surely you noticed that Metamagic buttons are on fix position and cannot be removed, nor moved, or hide ... therefore there is no searching.

I admit i would personaly preffer them to be on some special row "abowe" the hotbar ... but Larian seems to want them on right side.

Will be useful for multiclassing, but I don't know if we will ever get it.
It has ben confrimmed by Swen himself ... even tho it has ben some time ago. :-/