I'll agree that just logging in to keep the account doesn't make good use of it and some sites have user limits, so it hinders others from getting in. But sometimes you just get busy in life. Hey, you've bought a new house, sold your existing house but the new is delayed one you have to live somewhere temporarily, or you go on a trip for a month, stuff happens. Or you cancel your seedbox for a month or two.
Even
PTP says it themselves: Get the word out to your friends.
I think its OK to not use a tracker for a while if you get busy in life. But there is a balance between that and just holding it indefinitely which might prevent another user from joining. Trackers could prune accounts that don't DL/UL as well if they really wanted to.
Anyway, in summary, it is nice to let people know. Some have legitimate situations, for those people I'm sure they would appreciate the heads up.