The problem is that it’s actually less motivating to get paid a small amount than it is to get paid nothing. So it probably feels bad to put up a node that’s rarely used even though, when it does get used, it’s especially appreciated.

(Bad ideas in this space: GRCoin which pays you for folding@home but can never actually pay the power costs, Payver which supposedly paid for capturing road images while driving but ended up giving like $2/year.)