First seen in "Star Trek III", the Oberth was described as a science vessel. She seems to have very little armament, and certainly has very weak shields - the USS Grissom was destroyed by a single hit from a Klingon Bird of Prey. I've pitched them as being simple, low cost ships that would be built in great numbers. The idea of the engineering plant being unmanned is largely due to the fact that there seems to be no easy way to get from the saucer section to the engineering hull - the nacelle struts look a little narrow to fit a turbolift. Given that these are not combat ships, I don't think it likely that the designers would worry much about the crew having to repair battle damage or do much else beyond routine maintanence every now and again.

The idea that the Galaxy class was to replace the Oberth comes from the Tech Manual (page 1), but I thought I'd give these ships a stay of execution since we've seen a couple of them around during TNG.

Although many fans seem to love the big, heavy-combat starships - and I do too - the Oberths probably better represent the ideals of the Federation than almost any other class.