Here is an excellent article on RAM testing - including the philosophy behind the tests:
http://www.netrino.com/Articles/MemoryTesting/index.html

Whilst I agree with the misgivings already expressed about the value of memory tests as part of a product (especially internal memory tests), tests such as these are invaluable during development.

I recently came across the above-mentioned tests while proving the memory design on a client's new hardware - and they did reveal some marginal timing issues which had not shown up in the hardware designer's memory tests.

So, while testing the internal RAM and ROM is probably pointless, you should certainly apply a rigorous test to your external memory design during development - and possibly also as part of your production testing.