What about pro rata expense sharing? For a recreational flight with no transportation motivation, wouldn't the common purpose be "recreation"?
Maybe.
The problem is that the regs have been interpreted in such a way that almost anything can arguably come under it by just tiny changes that are in the real world pretty inconsequential.
For example, does the rule change depending on who asked whom? Well, if the non-pilot asked the pilot, we get into the "wouldn't have gone otherwise" question that gets bandied about in these discussions.
At some point most of come to the "oh, give me a break!" break point where you look at whether a situation is the flip side of the duck test: If it
doesn't quack like a duck, it's not a duck.
My personal favorite:
Your sister wants to go to her best friend's wedding and asks you to take her for the share of the costs (oh, heck, have her pay all of them!). You are not going to the wedding and, try as you might, can't come up with an independent reason for being there.
Would it be a violation?
One could technically argue "yes" based on existing Opinion Letters and NTSB precedent.
But really. Does anyone think that anyone, including the FAA would give the proverbial rat's behind?
These discussions can be very helpful in highlighting some of the lesser-know intricacies of the private pilot compensation rules. After all, since ignorance is not a defense anyway (even though it may be bliss), it usually makes sense to understand the rule before deciding that it's okay to violate it.