My players will tell you Spot and Listen are the key skills, although I do make sure that all skills are utilized over the course of the campaign (esspecially knowledge skills). But spot and listen come up all the time. Ambushes, little details that only flash by vision for a moment (like the keyhole in Lonely Mountain from the Hobbit), the sound of footsteps or other distant important things, overhearing conversations, chasing pixies... the list goes on and on. Not a game goes by where multiple spot/listen checks aren't made.
Climb and balance are nice, but can be replaced by spells such as fly. Spellcraft is important for someone in the party to have (probably the arcanist), but once one person has passed the check, it's a free action to communicate this to everyone else, so I wouldn't call it a must have for everyone.
But Spot and Listen can't really be replaced by magic very well (I mean, Alarm can help in some areas, but not for everyday needs) and since the moments they're needed in can be so crucial, you don't want to have to rely on just one person for it, who might fumble the roll. More dice with good mods means better chance. And besides, sometimes, when it comes to ambushes, those with the spots or listens don't get surprised, while the others do...