Because it needed to see. By the way, nice job stealing the King of Fucktards title from Mr. I Can't Spell Samurai. He never saw that coming, mostly because scouts actually do sneak past incompetent fuckwits, even though they fail vs everything else.
No it didn't. It had Mindsight, remember? That was the only thing it was using to spot the scout in YOUR EXAMPLE. And in YOUR EXAMPLE that's enough for it to attack blindly. The fact that its head was revealed was the reason it glowed like a beacon, yet that provided no benefit to it whatsoever because it couldn't spot the stealther anyway with vision and was relying solely on Mindsight. Thus, its head being revealed was stupid. Had it been under the snow completely, it would have ambushed your party because they have no decent scout.
So once again we come to Sunic's logic failure: you have to pick one of these two things.
1) Mindsight is enough for the ice devil to pick a target and attack. If so, it can maybe jump the stealther (though the stealther, who almost certainly has better initiative, should be able to avoid this attack). However, if this is the case it should not under any circumstances have its head above the snow (since doing so provides no benefit), and thus should not have been glowing via lifesight, and thus it should have been able to trivially ambush your actual party of Clerics and Wizards. This means the only reason they didn't need a scout was because you were playing the mob extremely stupidly.
2) The devil felt the need to actually see the target with more than just Mindsight. If so, Lifesight can pick it up... but detecting a stealther with mindsight alone wouldn't be enough for him to attack, so he could never possibly jump the stealther. As such, the scout is completely and totally safe in this scenario, and everything you've claimed about how vulnerable that scout was is a lie.
So which is it? Or are you going to go with the usual "Hi Welcome Herp Derp" response because you don't have anything better? Right now in Sunic's world, the Ice Devil "needed to see" and yet would gladly attack without needing to see, and thus despite knowing about the party and where they'd be it revealed itself like a beacon for absolutely no reason. In every scenario you've given for why scouts suck, scouts would have been extremely important. A scout with Mindsight or Lifesight would have completely negated your guards behind a wall scenario (because they would have instantly popped up on both senses) and, if you'd actually played this ice devil with any hint of intelligence, would have picked up on the devil too quite safely. How about you try again with less fail?
Here's the truth about scouts: good scouts keep the party far enough back that they're really hard to detect, while staying within the party's engagement range (the higher level you go, the further back the party needs to be but the longer the engagement range, so this is easy at all levels). As such, he's not splitting the party any more than walking 20 feet away splits the party. They let the party know about the monsters before the monsters know about the party allowing the party to get 1-2 actions before the monster can do anything, which in a game where actions are everything and fights often last 4 or fewer actions is critical. They have all the same autodetect abilities that monsters might have, and a scout has them all the time while only a few monsters will have any of them, so they know when a monster autodetects them in the rare case where that happens. They have ablative defenses (such as Moment of Perfect Mind) so that they're the best person to take one or two hits early, in case that comes up. And they have far more reliable detection abilities than purely magic ones, which are already trumped by spells like nondetection and Mindblank. And they're in constant contact with the party by whatever means available (I prefer telepathy, but that's just the characters I tend to run. There are plenty of options).
In the end, the question of whether a scout belongs in a party is this: what's better, a party of 4 casters who have all just been hit with AoE debuffs or ambushing single target kill/suck strikes in the surprise round, or a party of three casters and a stealther who have just launched three kill strikes or debuffs at the enemy in the surprise round? Tromping around together in a tightly packed group blundering blindly from encounter to encounter just means you get ambushed and hit with AoE attacks, if the monsters are actually playing smart.
JaronK