Yeah, I've never had a problem with Multiclassing... because I don't trust Multiclassing and generally do my best to convince my players not to do it. When I've allowed it, it's reluctant enough that players know to avoid cheese, or they'll never get to multiclass again. (Okay, I don't actually control what my players do all that much - because I play with people who play nice with others, so I don't need to police them like that.)
Still, your point stands, I think. Multiclassing would be encouraged more across the board if DMs felt it was "safe" to allow it.