I have housed guinea pigs outdoors for the last 7 years and they have always been fine.
It is only very recently that my current piggies have become indoor pigs and that is purely because I moved to a bigger house last October and was caring for an elderly / poorly piggy from Christmas up until she died in March so my remaining 3 never moved back outside. This is because I now have the space to house them indoors in a separate room that doesn't affect my partners allergies so much.
Up until two years ago, I housed piggies outdoors all year round in a well insulated hutch complete with a rain and wind cover until I decided to move them in to a free ranged shed.
I think that guinea pigs can live outdoors perfectly fine so long as proper measures are taken to keep them warm, dry and safe in the winter as well as cool in the summer.
The only thing that I must add is please make sure you have a spare indoor cage available that can be used in the unfortunate event that you have a poorly piggy and have just enough room available so that outdoor guinea pigs can come indoors if needs be during very bad weather.
I really do not think owners should feel bad for having their pigs live outside so long as they are well looked after and cared for x