I hear this all the time... "what is there to gain by saying anything bad about an ex-teammate?" Of course, you only hear this from those that are fans of the ex-teammate. But then, what is there to "gain" by saying something positive?? It has NOTHING to do with "gaining" anything and is purely a guy answering questions that he's asked, and doing it honestly.
You have to also take into consideration that DT and the rest of the team, have been asked about Tebow 10,000000000 times since he was added to the roster. DT didn't say anything when Tebow was here, but probably WANTED to. Remember, Brandon Lloyd said something on his way out about Tebow as well. I think its pretty safe to say, that DT knew (or felt rather) that he was pretty much speaking for a LOT of guys in the locker room. Saying things they couldn't say before, because Tebow was in the locker room. Give him credit for "keeping things in the locker room" when it mattered.
But you asked "why talk about an ex-teammate?" Because he was asked...again... just as he was asked last week, and the week before, and the week before, and last year... and all last season.. and the season before. After a while you just want to say "Hey, being a receiver wasn't great..." and it probably felt good to finally say it.