sabotaging one dip-class perk isn't really fair, unless we start taking on all the others. (I don't recommend this, BTW, because where does it end?)
It isn't that all one-class dips are a bad thing, it's that some classes have features that are
too good frontloaded into the first few levels. Dipping monk on a druid gives you potentially around 20 AC for only 3 levels between the WIS bonus and tumble.
Dipping 3 rogue on anything gives you 6 AC and access to every spell item you could ever want.
Dipping 3 bard on anything gives you the same as rogue but +6 to saves vs. spells and discipline as well (and more, if geared for spellcraft).
Dipping paladin on a sorc gives you fear immunity and +14-15 on your saves.
If we just reign in some of those crazy powerful dips (without making them totally pointless to take!), it would go a long way to addressing power creep. Asking for this stuff isn't a matter of being against multiclassing, it's disagreeing with the enormous scale to which it increases your power in only a few levels.
Cap monk AC to the number of monk levels (maybe 2/level), cap paladin/blackguard CHA save bonus to the number of paladin/blackguard levels (maybe 2/level), and cap UMD/tumble to the number of rogue/bard/assassin levels +3 (maybe levels*2+3). That would still allow someone willing to take 6 or 7 levels in a class the full amazing bonuses, but give a lesser bonus to someone who takes only 3 or 4 levels. Importantly: people building their characters would have to really think about whether they get more from a 5th, 6th, 7th, etc. level in their dip class than they would by taking more levels in their main class.
As it stands now, once you take a 3rd paladin level as a sorcerer, the idea of taking any more paladin levels is laughable. This could make you actually think about that option seriously.