My take on this is that it's probably not a place we can draw a bright line and say "This is always okay, this is not." Theme camp fundraisers, local regional events, etc. are obviously welcome. The Burning Man calendar is moderated, but events that are of interest to some burners are welcome, as long as they are not blatantly commercial. I'd feel like ePlaya is sort of the same. If someone shows up and they've never posted before and they hit and run with a party invite, I'd find that undesirable, but I don't think every event has to be an official one to be shared.
Why are events okay if products aren't? Because events are an opportunity to gather together and continue the Burning Man community (drink!) connection throughout the year. Only certain events affiliated with the Regional Network may call themselves "official", but other events may be of interest to people here, and may be a chance to keep the connection going.
So, if there's not sponsorship, if there's some element that may be of interest to burners (a poi class, a fire-barrel construction day, a local picnic, or perhaps a party, yes) if the public is welcome (no "email me for secret directions") and if there is no commercial intent, I think we can allow those things on the local announce lists and on the Calendar, and I don't know that there's reason to keep them off the eplaya.
I think they belong in Regionals and Events, in the folder specific to the event's location (and not cross posted!) and I think a close eye should be paid to those areas to keep things on track.
OTOH, if promoters start to look at the ePlaya as the perfect place to post their parties, but they're not at all involved with the community or only ever post to advertise their events, I think it's spam, pure and simple.
And in some cases, I really think the voice of the community (drink!) has to be the deciding factor. Our policies have really been about commercial posts (selling your RV or your playa flashlights) but events are a less easily-defined area, because sometimes they're truly not commercial. edit: By this what I mean is, if something seems off to everyone, it's probably off.