I believe going for DeCastro is a lateral way to gain a better LT as we've been discussing. If he's within 3 spots it's worth going up slightly to gain him. I think there is a case to be made with Cwill at LT. He's gotten better, had to play more on his heels, has been more physical, and was helping Webb do his job quite a lot from what I've seen,heard,and read.
You put Cwill or Carimi in there at LT with the masking or the chip help and your really saving Jay's bacon. DeCastro will be not the best "impact" player but will make the greatest indirect impact towards solving the weakest foundational spot.
Good arguments for and against DeCastro. Personally I think he'll be gone in the first 15 and trading down for him is unlikely. If he was there at #19 and my other choices rated nowhere where he does I'd take him but I wouldn't give up a 3rd round pick to move up to #15 to get him. He'd make a difference in the line that's true, but so much in the overall scheme of things. We need impact players at other positions more than we need an OG.