I think that when you're where the Celtics are in the draft, you have to pick according to need. Buuut, when you're at 30, you're not going to get anyone to fill a need on a nightly basis. Douglas-Roberts is wasted on our team. So is Brandon Rush. Are either of these guys going to beat out Posey for minutes?
I agree that need should come into the equation at 30 (unless there is a talent much greater than the others available). However, I believe that the C's do have a need on the wing.
Currently they have Pierce, Ray Allen, Posey (although he could opt out and be gone), and Tony Allen.
Personally I think Tony Allen is on his way out. He has not improved much since his rookie year, and he is running out of chances. Not to mention, I believe that CDR, Walker, and Rush could all be better than Tony almost immediately. You are right that they won't be getting minutes over Posey, however, in an 82 game season you need more than 3 guys for depth in those two positions (SG, SF). Especially since Ray Allen seems to be breaking down a bit over the minutes he played this year, and IMO could really be helped by dropping down to 30 minutes or less per game.
If we are able to get a talented wing (like the 3 mentioned) who could come in and provide depth, and scoring off the bench, then I think that is absolutely filling a need.
As the Celtics are an aging team, they need someone who can come in and provide some certain offense. Notice I didn't say "instant." Hibbert is like a plodding fairy tale giant! But he shoots 60% from the field.
Hibbert - or any offensive-minded big man - would add a new dimension to the team.
See, my problem is that I don't see Hibbert as "Certain offense". I feel that because of his lack of quickness, and his mediocre footwork, he would really struggle to get quality shots off in the post against NBA defenders. He is not a great outside shooter, like someone like Z, so he can't really hurt defenses that way, and although he is a good passer, he isn't extraordinary.
Basically I see him as being a guy whose only skill that is above average is his height. Everything else is average at best (shooting, passing), or significantly below average (defense, footwork, quickness).
And finally, I think that you can't discount the fact that he is a horrible fit for the C's defensive scheme, which forces big men to stray far from the basket, and be able to stick with quicker guys on the perimeter. There is about a 90% chance he would never even see the floor for the C's because of this.