The problem with the logic of Celebi at least, is that Ice Beam is going to be very common considering the stat spread won partly on the basis of countering Salamence with Ice Beam, and that was even before the abilities were in. "proxy countering" is a huge issue given the offensive threats we want CAP10 to switch into.
Swampert I can see requiring HP Grass, but Celebi is a substantial threat anyway on the basis that defensive Celebi carries Thunder Wave. If you run Bug Buzz to deal with it I don't see the problem, because Bug is generally an inferior attacking type to Ice. If you run Surf/Thunderbolt/Bug Buzz/Support you're susceptible to Salamence again, if you run Surf/Thunderbolt/Ice Beam/Support you risk being paralyzed or KO'd by Celebi. There are 4th moves you could put in to address either (including the other move), but that has its own weaknesses.
Absenting these different coverage moves does not make countering walls more likely, it makes it less. If we limit CAP10's movepool so much that there becomes only one truly viable physical and special set, we remove the element of 4MSS that inherently comes with customization.
Lets say Bug Buzz is an option for this pokemon, and you need a set that can handle Latias, Celebi, Shaymin, Gyarados, Swampert, and Tyranitar. You have a lot of physical attackers so Blissey isn't an issue, but you'll be running Specs because you want to hit hard. So you select Surf, Thunderbolt, and HP Grass to deal decent damage with STABs and get rid of Swampert. Now you have to choose your last move. Bug Buzz will guarantee you a KO on Celebi while hitting Latias and Shaymin for similar damage to Ice Beam. You will also be able to keep going if Tyranitar switches in, even if less effectively than Surf. Ice Beam does better against Gyarados, but leaves you with only one move for TTar (HP is lulz).
In the absence of Bug Buzz, you can always rely on CAP10 to be running Water/Electric/Grass/Ice for the purposes of this set. Remember our concept is to explore defensive versatility. If there is only one set available because we've gimped customization options, we don't really have defensive versatility. In this scenario you are basically choosing between letting Gyarados come in on three moves or letting Tyranitar come in on three. Without that choice you are predictable, and all the Tyranitar user has to do is bait moves not named Surf.
I realize this is somewhat contrived, but I don't think Celebi is a good example for our purposes. I would rather use something like Bronzong, Swampert, or Jirachi who, in the absence of a non-hidden power SE move actually have a chance to come in on most sets. CAP10 still will not be able to counter everything at once, but it will expand it's defensive versatility, a part of the concept I think needs more exploring.
Addendum: The other problem is that not all Celebi are defensive Celebi, some are quite offensive and Bug Buzz is needed to take those down before CAP10 gets clocked. Pokemon that are that versatile present a challenge in that they can be both offensive and defensive threats, and it's hard enough to manage those without taking into account Magic Guard's penchant for allowing boosted no-downside damage.