Most pro gigging players use piano as their main board. Thats why you see new models that are compact and light like the new Roland FP88. Then the add a second keyboard for the others stuff like the Roland VR09. You've got enough sounds to handle most anything.
Theres also students and people who alwasy liked or wanted a piano at home but didn't wnat a big piece of furniture. Its also nice to have something they could play with headphnes.. The music store I briefly worked at sold a ton of guitars and a good number of lower end DP to that crowd. Wish they had them when I was in College so I didn't have to go find a practice room everyday and could play in my dorm room. And also just about every College Music Major is required to learn at least a basic knowledge of piano so the schoold put in labs.