Well, lots of people, or perhaps a few people with logorrhea, can, if the comments on this LL blog entry are any indication. Looking over it, the objections, I think, fall into the following categories:
What am I up against, Father?
EEEEEEEEVIL! -- Richard Burton chewing the scenery and spitting it out, Exorcist II: The Heretic
Mesh is EEEEEVIL! People who now comfortably sell cheap trinkets made of prims will be turned out into the virtual street by an evil cabal of people who know how to use horribly complicated software that the rest of us can't be expected to learn...aaargh, opensourcenik technocommunist wikinista FIC... [exit stage right accompanied by people in white uniforms]
So we have a tight little evil cabal... that anybody willing to take the time to, you know, actually learn something can join, just as they had to to wrangle prims.
Protectionism with a Human Face
These folks don't object to meshes; they object to mesh import... but that would require Linden Lab, after massive layoffs, to roll its own mesh creation tools, so operationally what's the difference? As long as SL remains a walled garden for content, they can be a big fish in a small pond and keep potential competition out.
Mesh shouldn't be allowed in SL until there's a way to guarantee stolen meshes can't be imported. (I have to wonder again whether there's any practical difference between that and mesh shouldn't be allowed in SL, period. DRM is a futile effort; vide the recent total cracking of HDCP.)
These folks I actually have some respect for. Yes, in a perfect virtual world there would be in-world mesh creation tools. There's a lot to be said for being present, in a sense, with what you're working on. It gives you a feeling for scale. Other people can be there while you do it and
(Heck, in-world creation can even be art in itself. If you haven't seen Robbie Dingo's magnificent machinima "Watch the World", do it now!)
That said--again, Linden Lab doesn't have that many people, now at least. Existing 3D creation tools have a whole infrastructure of documentation, tutorials, a user community, and a bunch of people concentrating on that tool who've put more effort into it than LL ever could into a home-grown in world mesh creator. Who do you think can produce the better tool?
Say you're Linden Lab. EVERYONE is complaining about lag. Mesh will make Second Life a visually far more interesting place, and will seriously help with lag. What do you do: allow mesh import so people can receive the benefits of mesh, or delay it until, with far less resources than you had before, you can churn out in world mesh creation tools?
I'm an immersionist, heaven knows... but I think Linden Lab is doing the right thing by allowing mesh import.