I always try to encourage my players to "play what they want, damn the rules", but this player has been better than all the words I can spew about playing "unoptimized" choices, because he's done it and shown us all how much fun they can be - even sometimes, in failure.
I encourage my players to do the same, and I find the best way to do so is to assure them that I am an ACTIVE Dungeon Masters.
When I read posts here I get the idea that most dungeon masters just sit and watch. If one player makes an "optimized" character and the other player has a weaker sidekick character then I believe it is the DMs job to make sure everyone is having fun, and not just sit and watch.
There are lots of ways to "make sure everyone is having fun". They could involve NPC's that give story arcs to the weaker PC with great reward, they involve items that boost the PCs power, they involve on the fly houserules that say no when you use that power you use two dice because you drank from that magical fountain.
It also involves making adventures that focus on everyones strengths, if someone chooses an illusionist, it is a good DMs job to make sure its not all undead all the time.
People always respond to this by saying "I dont want a character who forces the DM to break the rules to make me stronger" to which I reply good DMs are ALWAYS breaking the rules, they break the rules for everyone. They break the rules when the monsters don't work the way the book says so (to suprise players who always use their metagame knowledge to know the weakness of a monster), they break the rules when they cause monsters to flee, not because of a morale check but because the fight was feeling to long or to serve the purpose of the story. They break the rules when they make cool and different houserules, or change the way magic works forcing you to use "gestures" at the table, or give you a book to turn you into a lich, or make their own magic items cause most of the ones in the DMG are lame.