The people -- who typically don't play those characters -- saying that it's fine that the character can't do anything but offer advice or throw darts are basically saying that it's OK for one character to stop being fun after doing their one big thing.
I
do play casters- full/half/pseudo; single classed and multiclassed- and I never saw a problem with having to ration spells/powers.
Part of that is I play somewhat like my aforementioned friend (though not with anything approaching his skill*).
But also, my mindset is that
every PC has times when they can’t do what they’re awesome at doing…
in any RPG you can point at. So you need to have thought of other ways to contribute; other ways to have fun.
Imagine, for instance, playing a sniper in a modern/sci-fi game, and the session features mostly diplomacy and espionage, and the only combat is close quarters.
Or a melee specialist in a combat featuring flying foes you can’t reach. (Saw that happen at a “gameday”.)
Hell, ever played Shadowrun with someone playing a Decker? A lot of times- especially in 1Ed, game night was a binary situation: either the Decker did almost nothing they’re skilled at, or it was an episode of “That Decker Show!” while everyone else was a spectator.
* because I’m not always casting the same spells
