No, a single mouse movement doesn't matter. But when you're in combat with six characters, the character panels located on side, and the actions along the bottom... you can end up with a lot of screen traverses per combat round. You move to select a character, move down to switch to bow, move up to select the target, move to select another character, move down to pick a spell, move up to choose the target... It all adds up to lower efficiency and greater tedium.
hmm... to be honest, when I played BG1&2, I've never done it that way. Even now, when I play BG EE, I choose a character by clicking on it on the 'game screen', not by clicking on the portrait. Or simply by presing key-shortcut: 1,2,3...
To me those portraits are for:
1. how much damege was dealt to my party member.
2. party member status: blessing, speed, labirynth..
3. switching betewwn characters in 'inventory mode'.
4. maybe something more, but I can't figure it out now...
You don't have only one way to select one party member or another, and those portraits are not only for one thing.
Seriously, I want them to by 'visable', not mini-hidden-icon-in-the-corner-of-the-screen, just because 'it mists the battlefield'. One can move his camera to see what is there, it's made for this.