Well they arent all very nocturnal. Mystique is believed to be a dwarf and she comes out at all hours of the day. I love Mystique. For those unable to keep larger tanks dwarfs are the only viable option.
I remember a college student who was unable to have any tank larger than 10 gallons... so a dwarf would be perfect in that case.
With mercs being able to keep multiples is a plus. Also some just like tiny octopuses. I know when I watched Sedona it was so strange because she was SO tiny. Watching her was just fascinating. If you have never kept a dwarf you really cant understand and even if you do you may not. They arent for everyone. I wanted to stay away from them at all costs before I had Sedona. Now, I dont mind at all.
I will say it was very frustrating not being able to see her for the first 2 months. Then, when I could she only lived for a short time.