Are you kidding me?! A Fighter 20 is better than a Fighter 10 / Wiz 10, or even possibly better? You cannot be serious! Even the abosolute best Fighter hotfixes WotC put out, Dungeoncrasher and Zhentarim, cease giving out new boons by levels 6 and 9 (or is it 10?) respectively. 1st and 2nd level spellcasting alone is worth more than the entire last 10 levels of Fighter! 5th level spells aren't worth jack? Teleport, Polymorph, Greater Invis, Glitterdust, Alter Self, Fly.... Can't compete with an extra 5 BAB, 30 hit points, and 3 bonus feats....
I don't even really need to make a case if this is the counter-argument.
You seem to not understand the meaning of "level appropriate".Polymorph: You lose 5hd, but it is totally worth it: the best form you can turn into is a young gold dragon, which is fantastic! ...and it only costs 23000 gp in the form of a Ring of Dragonform, which will last six times as long. That's about 3% of your WBL, less than half of the cost of a +5 bonus.
Alter Self: You get to go from a 20hd warrior to a 5hd one-hit-kill. Congrats.
Glitterdust: Yay, you manage to make a grand total of zero creatures blind with your DC 14 spell. Waste of an action.
Fly: Better to use magic items, mounts, or party members who can cast spells worth a damn than a 10-minute, slow speed, easily-dispellable spell at CL10.
Greater Invisibility: At CR20, True Seeing is typical.
Teleport: Great, you have a way of fleeing from the fights you keep losing.
In addition, you suffer from increased MAD, arcane spell failure (unless you discard armour, in which case you suffer from a low AC), and not being good at anything you're doing.
Yes, Fighters suck. A lot. However, multiclassing spellcasters also sucks pretty bad. Neither of those facts is a good thing, and we would all rather it not be so, although as stated it isn't what this thread is about.