If someone has a reasonable thematic reason to do it...I'll allow it. However, if their "thematic reason" boils down to "I want to do 1000 points of damage in a round" then my answer will always be no.
And that's regardless what is in the books.
Now, don't get me wrong. I have no problems with someone developing a very combat effective character. But it should have a theme that relates to the character not the character class, skills, abilities, etc.
My players find out quickly..they can get away with a LOT more if they have a strong backstory, especially a written backstory.
If a player tells me...I want to play a Lawful Good Paladin/Cleric/Assassin....your backstory better be seriously impressive and I want a tight code of ethics from your church too.
But if a player just says, I want to stack these 5 PrCs, 3 base classes and 8 feats so I can get 14 attacks a round against everyone in a 10' reach...my answer will likely be ..umm, no.
Cedric