You know, when I made Jimmy Throckbottom, my 20th-level gnome who had levels in just about everything, I was pretty much joking.
Beyond the ramifications of HOW the character managed to get 20 classes -- I don't see him qualifying for many PrCs without the ability to get his BAB up there -- is how effective they'd be. I don't see him hitting very often or doing very much, spellcasting-wise -- although if you had enough of those +1 Spellcasting level classes, I guess you could add it up...
As a DM, of course I'd allow it, provided that the player could show me how they qualified for each PrC they took. I'd look forward to seeing if the player could pull it off, too.
Use, definitely. Might not be smart, but it's legal.
EDIT: Found Jimmy's stats and story. He's actually more Fort-Save based...
Jimmy Throckbottom was a tiny ol' gnome,
At a very young age the little feller lost his home,
When a big honkin' dragon swooped down upon same
And Jimmy's home town got swallered in flame.
So Jimmy hit the wild, he was small and wary and
For awhile he lived the life of a young Barbarian.
When a flock of giant wasps were in a stingin' mood,
Young Jimmy took his first big feat as Great Fortitude.
Jimmy's long adventures took him very far afield,
And over time he thought he'd sort of like to dual wield,
He was also old enough to think revenge was good to take,
So he levelled up in Ranger, favored enemy was drake.
But still there's more to life then telling dragons, "Arr, I hate ya!"
And all this travel gave Jimmy a passing love of nature,
So he made a change of heart, and some might say that he was fickle,
But he traded in his hand axe for a Druid's magic sickle!
By now our boy was known as being very hard to kill,
But a bunch of evil lizard-men were looking for a thrill,
So Jimmy he ran out of spells, and had to kill the blighters,
With his fists and blades and bonus feats, and so he levelled Fighter.
After fighting forces dark and killing creatures great and awful,
Young Jimmy realized that he was tending now towards Lawful.
He was sad to lose his rages, but when he sat down and thunk,
Ol' Jimmy, he decided he would rather level Monk.
Then a dragon, yep, the one who started this whole ballad, in
A place nearby was sighted. Jimmy levelled Paladin.
No ragin' and no druid spells, but it was still alright,
'Cause favored enemy stacks really well with smite!
Well, the battle lasted hours, but the dragon he got slayed,
And Jimmy fell upon his knees and thanked Mister Bright Blade.
For the rest of his long life, then, until he was deceased,
Ol' Jimmy fought for good and law, and did it as a Priest.
Jimmy "The Fort Save" Throckbottom: Gnome
Str:12(13 at 20th), Dex:16, Con:20(24 at 20th), Int:10, Wis:16, Cha:18
Levels:
Barbarian: 2
Ranger: 3
Druid: 3
Fighter: 3
Monk: 3
Paladin: 3
Cleric: 3
Saves:
Barb2 = F2, R0, W0
Ran3 = F3, R1, W1
Drd3 = F3, R1, W3
Ftr3 = F3, R1, W1
Mon3 = F3, R3, W3
Pal3 = F3, R1, W1
Clr3 = F3, R1, W3
Totals= F20, R8, W12
+ Con24, Dex16, Wis16
= F27, R11, W15
+Cha18
= F31, R15, W19
+Great Fortitude, Iron Will, Lightning Reflexes
Fortitude: +33
Reflex: +17
Will: +21
Ah, Jimmy. He could survive just about any kind of poison you could produce, but he still couldn't cast Fireball....