You aren't drafting a DT for the sake of doing it. You are drafting a DT because it's been a problem for you for who knows how long and you only have two on the entire roster that can play and one, Smith, I am being likely overly bullish on when I say he can play. There is a decent enough chance that he's a total dud.
No one is saying take a 5th or 7th round DT prospect at 56 either. If there are DTs available at 56 that you have high on your board, it makes more sense to take one of them as opposed to a RB, especially one coming off ACL surgery. If a similar rated LB is there, you take him over the RB. If a similarly rated C is there, you take him over the RB.
But sure, let's just keep repeating some of the mistakes of the last decade where we placed way more value in TBs than we should have.