I get kinda tired of people talking about how they only ever like the first 150 pokemon or that they only so as far as the gold/silver generation. They normally base this on looks and I will admit generation 3 does look like garbage but 4 and 5 look fine if not better then the first 2. The issue come from pure nostalgia.
I will admit to being an avid pokemon player. But I will never say that they are good games. They are always the same and the story was never good to begin with. You may have though so when you were 6 but were past that now and its clear that there bad. The first gen might even hold water because of now linier it is. There is far less simple running around stoping the bad guys then in later gens.
The advantage to playing with more then just the first gen is huge though considering the pool of pokemon. This is more of a meta game look at it but take ghost pokemon for instance...there is one, haunter. That's its. Not to say he was ever bad but he has one type and therfor while easy to counter has a place on every team. Same with alakazam. He's crazy good. Baring mewtwo probably the best in the set. And what's your counter? Scyther? There are no good bug pokemon to fight it off. Heracross was basically invented to fght him.
I'm not saying there are staple pokemon in the current meta game, scizor was on most teams because he's crazy good and there have never been a surplus of good fire pokemon/attacks in the game but flame thrower will kill him ourtight from almost anything. Same with ttar. He's rally good and bosts an immunity to psysic attacks. Combined with crunch just wins. The shere number of pokemon though help to curb this.
Next is the usefullness of abilities, scizor because good because of tecknition. No one uses swarm. The new dream world proves this with the ban on rain and possably all whether effects barring sandstorm and hail...because well, hail is garbage.
That being said, I play pokemon because I have this need to collect things and this is probably the best place to do it, elsewise you end up with 200 some odd ps2 games you are never going to play.