If you have limited climbing experience, is a guide completely necessary when summiting 5000+/6000+m summits in Bolivia/Peru?
Depends on which peaks. Something like El Misti might be reasonable without a guide, as long as you are an experienced trekker and make sure to acclimatize. For peaks in say the Blanca or Huayuash, for example, you had better know what you are doing, even on the "easy" ones.
It must also be mentioned that guides generally aren't that expensive in those two countries. If you want some fairly "easy" 5000/6000 meter peaks and are going in October (as your other post says), one suggestion would be to contact Zarate Adventures (click on the English Flag top of the page):http://www.zarateadventures.com/
I've gone to South America several times, mostly unguided, but guided twice. I used Zarate when I took my five year old son to Peru since I didn't want to head out there alone with my kid and wanted another adult along. A few hundred dollars will get you a long way using local guides (such as Zarate) and get you to several peaks.
Going unguided is very rewarding as well, but make sure that you know what you are doing, especially on the glaciated peaks. If you are going for big peaks in areas like the Blanca or Huayuash and are inexperienced, then you should really go guided. As mentioned, October isn't really the ideal time to climb in those regions either (which is why I mention the Arequipa area).