I had an original Edirol R-09 (non-HR) and got many years of good use and recordings out of it until the Roland R-05 was released and I bought one of those. To be honest, I don't use the internal mics that often, but I noticed that the overall sound quality of the R-05 recordings is better than the R-09. There is considerably less self-noise as well. Aside from the Sony's astounding battery life, the R-05 and PCM-M10 are roughly on par with each other, with the R-05 having marginally better internal mics (so I've been told) and a smaller overall size. Which deck is "better" comes down to personal preference.
Point of note that the Roland R-05, like the Sony, has also been discontinued and even used ones will fetch around $300 (they averaged around $200 new when in production).
Edit to add: To answer your original question, "Is buying an r-05 worth it for the internal mics?", I'm of the opinion that you might be better served taking the ~$300 or so you would spend on a used R-05 or PCM-M10 and putting it toward a set of lavalier mini-microphones and a battery box from a retailer like Sound Professionals. Better yet, buy a used set of mini-mics from the Yard Sale here in the community for even less money. They will sound far better than the internal mics of any pocket recorder I can think of.