It would be nice to have good defense and rebounding, but from the flawed selection of players that may become available to us you have to take some bad with the good. We accept Thomas' bad defense because of his offensive shooting and playmaking. Conversely, we should consider the cost to the offense of having a defensive-minded PF in there just to say that we have one. I'm not saying we should give up on defense, just that we were able to make it to the ECF without that type of player.
Now, don't get me wrong, since we are likely letting Johnson, Zeller, and perhaps Olynyk go and currently only adding Zizic for sure, we definitely need more frontcourt bodies. I just don't think that we can expect a really solid, all-around guy in that spot due to our cap situation.
In some ways, Ainge is paying the price for not balancing his picks out position-wise in the draft. By taking Smart/Young, then Rozier/Hunter/Mickey in 2014-2015, he created a lack of cheap big man depth on rookie deals that would be ready to play now. He addressed it last year, but kept Yabusele and Zizic overseas due to our roster crunch (and probably to delay their free agency clock). I understand that taking the BPA is smart, but I think that sometimes if you're picking late among a bunch of low-ceiling prospects, it's OK to let need guide your decision a little bit.
EDIT: Yes, Mickey is technically a PF but unfortunately he's not so good and I wouldn't expect a 2nd rounder to have a great probability of panning out anyway.