Nowadays, due to the decrease in the amount of work available, I'm seeing many more musicians double up on instruments to fill up the calendar.
Back when gigs were more plentiful, all I had to do was play bass. Singing of course helps, but not as much as it does nowadays as people want smaller bands to reduce the number of splits of gig money. I've taken singing way more seriously in the last 6-7 years as a necessity, not just for vanity. Often, that is the thing that I practice the most. Almost every pro horn player I know plays another instrument, because not all gigs require horns. I've been doing more acoustic gigs because demand for that has increased as venues don't have the capacity or desire for loud Rock bands. Most working drummers I know also play the cajon or djembe, so they can do the acoustic gigs. Also because of the rise in acoustic gigs, it seems like every drummer, keyboard player, bassist, horn player, etc. that can strum 4 chords with one octave of range are looking for these gigs. Same goes for every guitarist, keyboard player, bassist, horn player, etc. that can tap on a tambourine. I also see more guys doubling up on instruments on a gig as well. It's not uncommon for a horn player in a wedding band to also play percussion, guitar, keyboard, or different horns on a single gig.