I think that the basis for this criticism of pokemon stems from Yahtzee's admitted lack of familiarity with the series. It is an understandable, common, and easily forgiveable misconception. The "Gotta Catch 'Em All!" tagline was pushed extremely heavily during the first edition of games, mainly because it encouraged kids to play longer and highlighted the then-innoventive ability to swap pokemon between games. The "catch them all" concept was kept in subsequent releases because of how deeply ingrained the idea had become in people's concept of the franchise. After the 250 pokemon madness that was the original Gold and Silver however, Gamefreak began watering the idea down. In Ruby and Saphire they yell at you to complete the pokedex, but "completion" is kept within the context of that game's new pokemon. While it was possible to transfer pokemon from older games, the idea kept itself pretty much under the radar. I believe Diamond and Pearl took a similar approach, although I never played them so I can't really say. So far so whatever. It was in the re-releases of Gold and Silver that the notion of catching them all really began to take a back seat. In Heartgold and Soulsilver your stated goal, as given to you by the unspeakably overbearing NPCs thrusting their mandates upon you, is to beat the pokemon league (i.e. get all the bagdes and beat the Elite Four). As for the pokedex, Professor Oak still gives you the mission of filling it up but his dialogue is changed in an important way. He tells you that you should "Try" and fill it up with "as many types as you can". This is a big shift from "Must" and "All of them". It also shows the game cleaving closer to what the majority of people playing these games actually did. In Black and White the game's focus becomes even less ambiguous. The pokedex now no longer puts pokemon you have seen and pokemon you have actually captured into separate categories. All you need do to fulfill the "Catch 'Em All" sidequest (and it is now most certainly a sidequest to the main storyline) is fight each of the NEW pokemon in battle. This is quite an easy task to accomplish, or at least get far along in, without ever once deviating from the normal course of play. Furthermore, the device can't even record any of the old 400+ monsters until after the main game, which is elementary because none of them appear in game.
In short, I wholeheartedly agree that completionists are weird. My brother is one, although he genuinely does seem to enjoy getting everything, since its a means of exploring and experiencing every part of a game. He may be a mild case, however, since if getting 100% turns out to be miserably unenjoyable he stops playing. The pokemon games, despite their lack of radical innovation, must be acknowledged for both their continual minute refinements of the formula and for channeling those refinements in a direction which is increasingly consistent with the way that their games are actually played by their audience.