I you want to play a song that has elements of acoustic and electric tones, it is hard to switch guitars. The electrics with piezo allow you to get this done.
I have many electrics and two acoustics, one 6 one 12, both can be plugged in. I still want a p24 badly.
The piezo through a guitar amp is not good for me. I will use a DI box and run two lines, one to my gtr rig and the other to the PA. A Fishman Loud Box is also very nice.
Exactly. I play in a band that is mostly acoustic-based, but quite a few songs involve me soloing at some point or another, about half with some grunge/dirt (the other half clean), often going well above the 12th fret. So I use a HB with piezo. Being able to switch from that acoustic sound to a full on rock sound is really useful. I sometimes use the mag pups for those solos, other times just rock it out with the piezo (it works well, believe it or not).
I play the piezo through an acoustic simulator pedal into the PA for the acoustic sound (the combination really does a nice job, about as "acoustic" sounding as the other acoustic guitars plugged straight into the PA), and switch to a dirt loop (random dirt pedal, reverb/delay, and amp emulator) also straight into the PA. It does exactly the job I want for the gigs we play. The dirt "channel" isn't something I would use if I was playing rock'n'roll all night long, but works good enough as counterpoint to the acoustic stuff.
The piezo needs to go direct to PA for maximum effect - if sent to an elec guitar amp it ends up sounding too "electric" - 'tis the nature of elec guitar amps, even their clean channels - they are not a flat EQ response! If it made sense and I had the room "on stage", I'd send the acoustic signal to the PA and the dirt/mag pups to a proper elec guitar amp, then mike the amp for the PA (if required), but we simply don't play venues where that would make sense - too small, too intimate, I suppose. Which says we don't exactly play to huge crowds, I suppose...
So yes, a piezo is very valuable to me. I used to play with mainly an acoustic guitar, swapping it out for either my CU24 or an Epi Casino for the songs with leads, but the HB with piezo just covers all of that territory ever so efficiently.