I voted "no". I'm willing to Rule Zero a lot of flavor restrictions if someone brings me a good reason or a good backstory, but I think that the restrictions are a good default for a campaign.
For instance, the Monk and Paladin restrictions reflect the idea that these classes take their calling very seriously; you cant just wander away and pick up a couple levels of Rogue because you feel like it. Master Po will boot you from the Shaolin Temple when he discovers your disgraceful lack of discipline. So I understand the flavor reason here, it makes good sense.
Basically, what I worry about as a DM is people taking classes or races and then playing them as a generic jumble of stats. "I am a Hobbit because I want +2 DEX". Umm, yeah, thats great. "I am a Paladin because I want a special Warhorse at 5th." Grrr.
As such, I'd be more willing to let a Monk multiclass into Sorcerer if a player were to write up a couple paragraphs about an interesting organization thats devoted to blending the fighting discipline of the Monk with the mental powers of the Sorcerer (and how her character came to discover this organization). But if she just wants it "because then I can cast Mage Armor on myself" then I will say "nope".
-edit-
It is easy to get around the monk and paladin by taking what ever levels you want before or after the main class...
This is a good example of the sort of thing I am talking about. If you try this trick in my campaign Master Po is going to have some sort of interview with you before he lets you join the Temple. If you are sincere in pursuing the Path of Monkly Enlightenment, you are in. If you just want to get Stunning Fist and Evasion then you are out on your ear. (Although there may be a LE Temple somewhere which is not so choosy. ;-)