I use to be a 100%-er of Pokemon games...kinda...sorta. I didn't care that they had to be all on one game. As long as I had them within the right generation, I was fine. So I had 150 original within Red, Yellow and Blue. I had a checklist to make sure I did, marking what game the Pokemon was in. Then went it went to 250, I got the new 100 in Gold, Silver and Crystal, meaning I had 250 within the six games. Once again, I had a checklist that told me which game each Pokemon was in. This even stayed true for the third generation. I didn't mind that I didn't have 380some in one game. I had them in the nine games, and that's all that mattered, even though you couldn't transfer first generation to third generation. Then came the 4th generation, where 110 more were added. Around the same time, I had found out my original Red, Yellow and Blue, for the original Game Boy had died, and my Gold, Silver and Crystal were dying (probably the reason they made FireRed, LeafGreen, HeartGold, SilverSoul). At that moment, without those original 150, and having trouble to catch the latest 110/120, I viritually gave up trying to catch them all. I just play Pokemon for the main story line. But when the main story line, in the basic form, is very repetitive, I wonder how long I'll stay hooked into the series. Maybe this is why I don't feel rushed to buy Platinum, HeartGold/SilverSoul or Black/White.
The other reason why I quit trying to catch them all was half of the "new" pokemon were "newly discovered" evolutions, which were a pain to get. Take Sneasel, for example (I believe I have the right example). To get it to evolve, it has to be holding a certain item, it has to battle during the night, and it has to level up to a certain level range during a certain time range. This is ridiculous! Even if this isn't the right example, I know there are other examples similar, even more adding on the ridiculous. I liked it when most only evolved by leveling up, some evolved by a stone, and only a few required trading it.