Because, because each team only has 15 spots and all Rockets PFs (short but strong/athletic) can play at center position well.
Might be a good deal, Deke has to be replaced at some point and Kristic is a better player than Woods, he has got a nice offensive game and while not physical he does have the length to block shots in the middle. We have 2 similar players now in Dorsey and Hayes so I would trade Hayes if Dorsey shows the potential to replace Hayes in summer league/training camp.
While Dorsey possesses many of the same skills that Hayes does, he is not quite the same player. Dorsey is a legit shotblocker, a skill that Chuck does not have. Also, Dorsey may not be quite the level of post defender that Hayes is. Dorsey is strong but has not necessarily shown that his fundamentals, positioning, hands, etc. on defense are near the level of Hayes's. While I think that Hayes is now more expendable with the addition of Dorsey, I wouldn't say that we should get rid of him. They are different enough that having both would cover a broader skill set overall.
Krstic could be a good signing because he can play the 4 or the 5. We'd have great depth up front with him.