Well, the same could have been said about Hachi, or Mr. 3 or Buggy etc. You never know when someone you thought was left far behind in the story suddenly becomes important again.
But Pell is hardly the only example anyway. There's plenty of characters that SHOULD be dead, especially because them surviving kills all the emotional impact of their supposed noble sacrifice.
Hmm, that just reminded me of something. I was kind of expecting Tom to be here in Impel Down. Back in the flashback they said he was taken to Impel Down. I don't think they ever came out and said he was executed, so I thought he might still be imprisoned here.