Doubling guitar tracks is a production decision and therefore should fit the production. For instance, if you're recording a traditional jazz group where the piano is featured, it might be a bit much to double the guitars because they will become overbearing and detract from the piano performance. It just won't suit the production, most likely. You may notice I haven't given any absolute answers there and that is because of the infinite subjectivity of recording and production.
Furthermore, I think you're confusing production techniques with looking for a silver bullet for a "better", or "bigger" recording. There are plenty of great rock recordings where the guitars were not doubled and other production techniques in the recording phase were implemented, such as room mikes, proper mike placement, a good sounding room, etc. Then, of course, there's the player and the instrument. If both are up to standard, a good recording is imminent.
Doubled guitar tracks are usually done for a wider, pseudo-stereo effect and often panned hard L and hard R. Doubled vocals are done for a chorus-like, thickening effect, which is similar, but a little different to the guitar technique in that its not uncommon for them to be layered in both mono and pseudo-stereo fashions.
At the end of it all, you may be surprised to find out that real professionals often keep it very simple and rely mostly on the tone of the source. Often you'll find them using a single mike, carefully placed in the most appropriate section of the room for optimal tone and "bigness". Whether or not the production may need a double is entirely up the producer.
Cheers