The reason he didn't get Ralts is that he's still a bit too young at the end of the season isn't he? I mean, physically, he doesn't seem to have grown much, and may have to wait a year or two before officially starting out. Unless I'm really missing something XD, but from the episode pictures of that last one, he didn't seem that old enough to be a trainer.
And I agree with much that has been said here. It helped develop Ash and May's characters. It may have made Ash and May want to set an example for Max, so that he'll follow in their footsteps and become a better trainer. He already has extensive Pokemon knowledge, and with a little help from Ash and May's battling techniques, Max has a decent head start.
It was also interesting to see Ash's mentor side, being sort of like an older brother to May and Max, helping them along the way. Ofcourse, Brock helped out too, but I think Ash's mentoring had a significant impact on May and Max.