what good is it to be a roleplayer if you're dead because your abilities dont match up in a fight with a straight classed character?
If you're intentionally trying to pit your Ftr5/Wiz5 against a solo Ftr10 in combat or Wiz10 at spellslinging, you're probably not playing your role properly. Your build is about situational flexibility, not going toe-to-toe with a specialist in fields in which you dabble. That's like asking a team of "weekend warriors" in baseball to take on a team of MLB all-stars.
PCs in our current RttToEE campaign have hit 10th level. My particular PC is a Ftr/Rgr/SpecWiz

iv/SpellSword. Due to occasions of enchantment, infirmity, injury (or other conditions) to other PCs, he's been forced into the role of spellcaster, trapfinder, and front-line fighter.
Had he not been able to perform those roles at
some level of proficiency, he or another PC would probably have died at this point. In addition, we probably would have left behind a device integral to success in the module due to no other PC in the party being able to use it (
at the time- since then, that McGuffin has been passed onto another PC).
look at it this way. a character with 5 levels in wizard, 5 in fighter, 5 in rogue and 5 in cleric sucks.
Mechanically maybe, but perhaps not so someone who envisions his PC that way.
To use another one, the multiclassed character is the decathalete, not the high-jump specialist, or the sprint specialist, etc., and there is absolutely nothing wrong with that.
if you are not good at ROLL playing, you won't last long enough to ROLE play. if you cant hit the jump shot from outside the key, what good are you to a team?
Perhaps I can block, steal and make assists. Perhaps I'm hot off the bench, but not good enough to start.
In every team, there are the stars and the support. The multiclasser is the support.
And for the record, I've never had a problem with multiclass survivability that would have been remedied by being solo classed.