I think is a good idea add a warning when chmod/chown is used instead of fperms/fowners.
The thing is, fperms and fowners operate on the install image in ${D}, while it may still be necessary to use chown and chmod in ${ROOT} for various reasons (including bug 141619 and bug 378451). So, would a ban on chown and chmod inside src_install really improve anything? Note that they'd still need to be called by Makefiles and whatnot.
I asked because I guess it can be an improvement, feel free to close if is not.
If it was a portability issue then maybe we could justify a warning. However, it's not, and I think a warning like this would just be annoying.