"Serious musicians only play one at a time"....that's a load. I guess Mozart hearing all those parts in his head simultaneously made him a hack??
For some(especially in the "look at me" MTV 80's) it may have been a gimmick, but during the height of album rock, concept albums, and prog rock, it became an esstial stage tool. Page playing a tune like Stairway that requires 12 string, then switching to the 6 string for the solo, then back to 12. Or like Mike Rutherford in Genesis, switching between bass and guitar mid song. And especially when bands perform a concept album in it's entirety where one song flows right into the next w/ no break, or at most a fast 1 count. Luthier brothers Hugh and Andy Manson from Devon England, while watching Led Zeppelin at Earls Court in 1975, saw their acoustic set stumble along w/ instrument changes between every song that also required repositioning the mics. (Ugh!) Without anyone asking, they built a triple neck acoustic for John Paul Jones w/ mandolin/12/6 string and gifted it to him before their 77 tour. He used it on every acoustic set that year, and it simplified things tenfold. No more bothersome changeovers, and the set flowed much better. And he definitely wasn't a "look at me" musician. Most of his life was spent in the shadows of "look at me" rock gods. Heck, even Alex Lifeson of Rush went as far as inventing a guitar stand that holds an acoustic guitar at standing player height/position so he could walk up behind it to play an acoustic section of a tune all while still having an electric strapped on. It's not all only about "look at me". Once the age of 16-24 track studios were the norm, the sky was the limit on what array of stringed instruments some folks included on a single tune. And many took full advantage, often using 4-5-6 tracks for various guitars w/ various sounds. For faithfully replicating it live, some of these dual purpose instruments became essential.