I tend to think that it's good to be very good at 1 or 2 skills for each character. For one thing, there are items that give bonuses based on how many points you have in a given skill, so being a total generalist doesn't help there. Also, Athletics is a good skill for any front liner to be solid in for the sake of the Second Wind ability. Furthermore, there are a good number of those both dialogs and story pages where there's a skill check. Some are checks against the talking skills, while others are for skills like athletics, streetwise, survival, or stealth. For any check that involves something athletic like climbing or swimming, athletics is checked, while if you're trying to identify something in the wild, perhaps survival is checked.
Overall, it doesn't hurt to have each party member be very good in a different skill. But it's also good to have every party member have at least a 2-3 points in Stealth so that the party doesn't get spotted a mile away.
If you like stealthily going through a dungeon checking for traps and disarming them, mechanics is good, but you also need perception to spot the traps first. I found that Eder as a rogue didn't really have the perception to spot traps, even though with strong mechanics, he could disarm them all. So I used Maia with her exceptional perception (and good stealth) to actually lead the way through dungeons, only bringing Eder up to do the disarming as needed.
Side note: Sometimes you end up in battles where there are traps in the middle of the battlefield. You can disarm traps in the middle of a fight, and I would regularly do so.
Background choices. Do I take them into account? When I'm making a main character, I probably do. I will say though that it's more important for mechanics than the other skills, but I happen to put a premium on always having a skilled traps and locks mechanic in my parties.