Yann: making it "logic only" would certainly be a lot simpler in the code. (Edit: did you delete your post? :P ) However I don't think your last example works. Remember if a single instance meets the condition the event counts as having run. So in your last example if *any* sprites have X > 35, the 'Else' does not run.
In other words going back to case 2:
<img src="http://www.scirra.com/images/else2.png" border="0" />
If 'Else' is logic-only here, and for the duration of the game there is always a sprite on the left of the screen, the 'Else' event never runs at all - it may as well be deleted. I think this is also counter-intuitive. It seems obvious what the event should do, so I think it's still a good idea to involve the sol-inverting feature.
I'll code it as option A (logic-only if a system condition is present, else sol-inverting) for first release, but with a warning it may change in future. I think having people play with it in real projects will be useful.