To be honest, I agree that SEO URLs should not be included in phpBB at all (not even phpBB4.). It's not an important feature, and it's not that much of a benefit. The people above posted a good argument about people changing topic titles, etc. I for one don't think phpBB should include SEO URLs as a feature, however, if somebody wants to make a mod or rather a "contribution" to the project that allows SEO URLs, by all means do it, and just like any other mod, board owners would choose to use it.
On my board I would not use SEO URLs, I got a fairly good rank as it is now with dynamic URL. I do have the User Blog Mod installed and it has a SEO URL feature, I do have that enabled, but thats not my board, its user blogs

.
So I vote no for the devs to include SEO URLs in phpBB out of the box.