Machinist is definitely the least played DPS role. Just going off FFLOGS you can kind of get the picture of how many more bards there are than machinists just on that website. I love playing my Machinist and can pull pretty good numbers and have cleared all of Alexander Savage. There are a few reasons why there are less machinists then bards.

1. Most bards just leveled bard to 60.
2. Bards Foe's/Battle voice is higher DPS plus Rain of Death
3. Machinist I feel is harder to attain decent DPS compared to bard
4. Machinist is slightly lower DPS on AS1/AS2 and the same on AS3/4 on average.

I'm sure there are more reasons like Bards attacks look cool while machinists attacks look like sh**. People don't like how Gauss Barrel looks, people want to play a real dps role (mnk/drg/blm/smn/nin), etc.

In my opinion while high level machinists can pull out pretty good dps at the very highest levels for most moderate to beginner players their dps will be really terrible on this job and with machinists high burst and worst sustain dps in the game that doesn't help at all. SE should do something to make it easier for machinists to sustain dps so less skilled players can pull out some okay numbers and then you'll see more machinists (and maybe give them some better visuals on their attacks because they are boring but I doubt they will fix that.