I think it fits with Daryl's character to go off with Merle. He didn't want to leave ... but he wasn't going to cut Merle loose by himself again.
First, Merle may be the only person on the planet that intimidates Daryl. (Rick might a little ... watching Daryl cower a bit when Rick barked at him right before the raid on Woodbury was interesting).
Second, he's family. They're southern rednecks. Family's family ... loyal even if you hate 'em from time-to-time. And, Merle turned on the Governor for him ... he wasn't going to be loyal to Rick over Merle after that.
Third, he gave Rick a choice to let them stay. Rick said no. He didn't leave just to leave ... he left because Rick was demanding that he cut his brother loose again.
Fourth ... I don't think he completely blames Merle for Glenn. It was the situation he was in because the Governor saved his neck. He know's Merle's a jackass ... but he also knows that Merle was left-for-dead, and owed the Governor something for saving his life. And, like Merle intimidates Daryl ... The Governor is the only one that inimidates Merle. Daryl knows what its like to be overpowered by a dominant personality.
Seems to fit.
It will break down one way or another. Either Merle will do something stupid that forces Daryl to turn against him ... or Merle will prove himself somehow, and Rick will figure out he needs all the help he can get these days and bring 'em both in.
SnakeDoc
Not to mention Merle, while a complete jack@$$, seems to be pretty loyal himself. I know a few rednecks who can't stand their family members but would still never turn their backs on them for anything. To these guys blood is blood. 
.
.............?????
