Is there a good reason why XMPP Ping support is not included?
No real reason why we do not support it. In general, we implement extensions for which we or someone in the community has a need. The number of extensions is so big that we are not trying to just implement them all. Having said that, I think that XEP-199 is a nice one to have although for a heavy loaded server I would prefer to keep the old whitespace ping if its enough for the problem trying to be solved. Unlike the whitespace ping this new XEP allows you to ping many hops but so far we never needed to do that. If you are planning to implement it either as an Openfire extension or as a plugin we would love to include it as part of the official release.
Check out JM-1414 , looks like support for this is coming in 3.6.0