It's virtually impossible to describe all the systems used by the most famous guitarists, mostly because it is impossible to know all of them, and in many cases are personalized ideas owned by them: if you happen to find a publication that covers this matter (personally I don't know if there is one), buy it because it would be very interesting. Concluding this long chapter, I want to remind you that knowing and understanding the instrumentation of others is always useful in learning new and interesting techniques. Besides looking at the hands of the guitarists, its always useful to observe how they use their instrument.

Conclusions

In many cases the requests of musicians can be influenced by various factors, such as the cost of the instrument, the type of music played, the ease of transport, etc...
Obviously, the ease of transport can be very important, especially if the musician has to load his instrumentation in and out of a car (not everybody has a van). In this case it might be useful to use a combo amplifier, or a setup made up of a small case containing a few rack units rack and a small speaker.
Others may instead opt for more complex systems (more expensive and bulky), where you are obligated to use of a van or station-wagon (unless you want to load it on the luggage rack!).
To make some good Jazz or Blues, all you need is an amplifier and a guitar, but to play pop, fusion, or progressive rock a more complex system is needed.
It's clear that an expensive setup, both in economics and the time of programming, is not available to everyone, and it's also clear that in many cases it only takes a little to get the perfect sound for that particular situation where you have to play.