we don't really have enough room for another guard.
Why? We only have 2 wings signed through next year (Pierce and Ray). Green could very possibly walk, and who knows what will happen with West and Wafer. This team is not set at any position going forward other than perhaps PG (and they could still use a reliable backup). They are really in a position where they should be looking at best player available.
We needa a big man badly.
Oh, of course, but when it comes to picking late in the draft, beggers can't be choosers. I would much rather have a quality wing than a big man who will be out of the league in 3 years.
The chance of finding a quality big man that late in the draft are very slim. It happens, but not very often. So, if there is a big man that they think will be a very good player, by all means, go for it, but if they think the best player there is a wing, then they need to draft them, and address their need for big men by getting a guy who already has a body of work in the NBA via free agency or trade.
Teams that use the draft to fill holes rarely end up happy with it.