I'm against it. The option to see the difference between files is, of course, a part of file management. But I don't think it is a good idea to make it a part of file manager.
* It could bloat the file manager ("bloatware")'
* It is too hard to competitive with separate utilities (
https://www.araxis.com/buy/prices)
Actually, I don't use built-in search as well. I use Agent Ransack for it.
It should be also said that a lot of developers don't use diff utilities to compare local files. Instead, they use Git or Mercurial or SVN or some another version control system.