With Bolden being a heavy contributor on special teams, showing a consistent 4.9 YPC throughout the past two years, making good progress in the blitz pickup, AND receiving balls out of the backfield last year (20 catches compared to 2 the previous year), it's not difficult to understand why Bolden is still on the team.
Much like you, I don't think he's anything special and I'd agree that his days are likely numbered if anyone better comes along, but in the meantime his understanding of the scheme and ability to do the things that are asked of him (and cheapness) make it more understandable as to why Belichick chooses to keep him around.
Mostly it's likely due to a lack of better talent, as Bolden probably wouldn't be here if Finch, Gray, or Houston had actually shown anything.