The slots are a great idea for several reasons.
1. People have GC controllers already. Having the ports kills the "OMFG I HAVE TO GET NEW CONTROLLERS WHEN I JUST BOUGHT A GC" whine-talk that people love to bitch and moan about. Nintendo wants to keep adding various peripherals (or so they've said) and then use them to make new gameplay experiences. The bongos and GBA connectivity are examples. I personally own like 6 wired ones (from a Toys R Us sale, paid less than 3 bucks a controller), a wavebird, and a Hori Digital Controller. If I ever run across a wavebird sale, I'll pick those up. It's nice that Nintendo is letting me continue to use them.
2. Sometimes wireless controllers fail. Corded options are always, always a plus because if, in rare instances, something fails, you have a backup option.
3. Ensures full backwards compatability with specialized games. I can't imagine how many people are going to bitch about how the PS3 can't play Steel Battallion (xbox game? I can't remember). Anyway, that would be one of the only times I'd think whining would be pretty feasible and understandable.
4. I dout it adds much bulk to the system.
5. I think the idea of a wireless multitap is nice, but that would be another accessory, people would lose it, etc. Better to have it on the system itself than give people the chance to lose it and then complain about "HOW COME IT WASN'T IN THE SYSTEM" over and over. People, as a rule, are stupid. Better to eliminate chances for any kind of conflict from the get go. (Aside: Does anyone else work tech support or in a call center? Imagine the bullshit people get and multiply it by about ten million. OMFG NINTENDO, WHY CAN'T I USE A NINTENDO CONTROLLER ON IT TO PLAY THE NINTENDO GAMES? I assure you many, many people will call in with that exact question, not realizing that the Rev's controller is capable of playing the older games. You laugh and think it's idiotic and there's no way someone could be that stupid, but my friends, you sorely overestimate the human population. For example, in my work, someone asked me if a game that required 1 gig of ram could run on 512. Another asked me how to resize a window. Another asked me which side the CDs go into the drive. And another asked me how to recharge an external battery, and subsequently got mad when I told him it would need to be inside the laptop and plugged into the wall. Yes, my friends, people are morons across the board, and it's always better to have intentions of treating them like they have a double digit IQ.)
I don't see a problem with having the ports. Hell, maybe they could make adapters to let you plug in SNES/NES/N64 controllers, although I think that's huge overkill.
Also, I'm a little curious about the wireless Rev controllers. I have to rule out RF, IR, and Bluetooth, since the machine doesn't appear to have those technologies (though, of course, no one is sure). But I doubt it would be wireless in terms of like an 802.11x protocol - there's going to be enough interference with a DS attaching to a wireless network, let alone the Rev. I'd speculate that bluetooth *might* be the final choice since it's such hot technology, and would leave the door open for future devices (and Japan, being a huge cell phone market, would eat up that functionality if Nintendo let people have that sort of access). But if the DS can connect to the Rev, it would HAVE to be 802.11x protocol, and probably a b network. Maybe b and g. I dunno. We'll find out soon enough I hope.