Good Question!
Well let's keep it a short sentence. Kevin Millar has not been what he was or use to be! Simple as that. With the Red Sox he would hit more than 15 home runs per season. With the Baltimore Orioles he would hit more than 20 home runs at times! Now why can't he pass 6 home runs this year?
Let's break this down a bit.
1) He's not an everyday player, rather a bench player
2) Age
3) With the Jays in no contention, he might not have that spark.

Overall Kevin Millar is not worth signing again! Not because he had a bad season but because the Jays are working around a
young team. Randy Ruiz will probably be the new DH next year and Lind back in LF. Ruiz hit 3 home runs in less than 2 weeks and that's half of what Millar hit in a whole benched season with the Blue Jays.
When Millar first signed with Toronto in the offseason as a Non-roster Invitee, I admit he was my favorite player, but once this stink happened with his bat, i changed my mind, just a bit.
I see Millar a power hitter with a team like LA Dodgers, Angels or Giants but not Toronto, at least not this year...We'll see what happens.
*-Steve