Because of the "convenience fees" you have to pay, normally it doesn't make sense to pay by credit card "to get miles" only. But is you can find an offer for something like "pay your taxes and get double miles", then it is more reasonable.

It may not be the best use (miles/$), but it is almost an acceptable rate! (IMHO - I would only pay by credit card if I can get double miles! Never at the 1:1 ratio.)