I can see an advantage to using a single subscription aggregator, and that would be bookkeeping, namely in keeping track of who is "owed" what level of rewards, and when you've reached milestones where you've promised additional features, but I also recall the advice of a well-published author to not put all your eggs in one basket.
Also, there's probably a bit of laziness in not wanting to go through the rigamarole of figuring out another providers service, their terms and conditions, and adding it as an option for, what at least initially, appears to be negligible gains.