I think Max is actually very important to the show, even though he doesn't look it. Like already stated, he is like a half of May.
May and Max are like a Manzia performance. May being the Boke and Max being the Tsukkomi. They go hand in hand together, and though they may be able to do it alone, it just isn't as fun and the jokes don't come through well enough I think. Max needs to be there to pull the joke through when May can't do it herself.
May's personality can only take her so far, but its only the characters around May that make her who she is, and Max has a main role in fleshing out May's character, since she is pretty one-demensional by herself like Chairman Kaga had pointed out. Max is the exact same way. His character is nothing without his sister there to watch over him.
Aside from May and Max being there to replace Misty, May and Max are also there to replace the part of Ash that deminishing as of recently. The childish, ignorant aspect.
Like already stated, now that Ash is more mature there needed to be a character who was inexperienced/ignorant... May and Max fit that role. Max is there to be a kid. Smart or not, he is still a kid and does kid-like things, which this show needs, since it is a kids show.
.... Or it could just be that the creators thought the show needed more loli- shota characters since Ash was getting more mature, and Misty was gone.