"I've got a deal for you. For $5, I'll chop off three of your fingers and give you a fluffy ball to throw at people."
"That's a terrible deal."
"No, it's a great deal. It only costs $5!"
It's a terrible deal. You're sacrificing effectiveness in your strongest combat options, in order to get access to a pathetic combat option. This is not a good trade, because you can still only do one thing per round; instead of having one good thing you can do, you have to choose between doing a less-good thing or doing a sucky thing. Either way, you're worse off.
For the combat optimizer, dipping for cantrips is really phenomenally stupid*. If you want to discourage dipping, go after the places where it actually matters, like armor proficiencies (the wizard who takes her first level in War-domain cleric to get heavy armor, a shield, and full spell slots, plus cure wounds). Or just ban multiclassing altogether.
[SIZE=-2]*The sole exception being the sorlock, who abuses the interaction of warlock fast-recovering spell slots, Agonizing Blast, and sorcerer metamagic to do obscene things with eldritch blast. However, there are much better ways to prevent the sorlock than nerfing cantrips.[/SIZE]