David, Elio, they are a great toy, and IMO have the potential to be an even greater tool, when used appropriately.
Few people, however, seem to reach very far beyond the toy level of use. It's too bad. Here are a few additional thoughts on how a guitar synth can be very, very useful.
Many patches require very precise fretting, string muting, and picking technique. Playing these patches until you get it right will improve your general technique tremendously. Piano patches can be especially tricky in this regard. You also need to use the synth option settings to help you out - turn off pitch bend, adjust triggering thresholds on a string by string basis, etc. It can be a lot of fooling around. Some single note instruments can also be tricky for soloing because you have to be very careful about string muting and inadvertent string bending, or you will not sound authentic.
Some patches just sound so cool that they can be inspiring, and you will probably find yourself playing lines you wouldn't otherwise play on guitar - this can be really good for expanding one's musical vocabulary. Synth sounds can open your mind to other musical possibilities. It will also give you an appreciation for other instruments and what makes them sound like they do and how and where they best fit into an ensemble.
A synth can be a fantastic tool for putting together backing tracks, or laying down multiple different parts on top of a backing track. As I recall, the Roland synths have some fairly good bass patches for a variety of basses, so for someone that doesn't have a bass, that can be a real plus. Similarly with organ / keyboard sounds. Or adding some brass "stabs" here and there in a track. Most altered chords (like 6ths and 13ths, flat or sharp 5ths, and sharp or flat 9ths can sound really cool with some of the brass patches. Finger pick them using small chords to get a unison attack sound of multiple instruments. The slow attack, long sustain string pads can also be very useful for filling out a backing track and adding ambience.
I have always been surprised that guitar synths are so rarely used in live band settings. They can be tricky and you can get unpredictable results, so maybe that is why the real pros shy away from them live. It would be easy to embarrass yourself.
Well, I do that all the time, so no biggie for me!
I hope this gives you some additional food for thought. Good luck and enjoy! Let us know how you progress. I'd be very interested in hearing your experience.