Hmm. Interesting discussion!
I totally understand where Artiste is coming from.
In our MLS, we are able to pull up and view the private listing info from neighboring MLS systems without joining them. We do have some reciprocity going as I can pay a small fee and enable my lock box key to open their lock boxes. We frequently overlap listings from one another - someone from a neighboring MLS may list a property in our area and vice versa.
It never once occurred to me that if I sell one of those listings from another MLS that I'd be entitled to less of a commission than if I belonged to the MLS in question. In fact, I had a house under contract from another MLS and was going to get the full 3% co -broke (unfortunately, the deal fell through over something else.)
It also would not occur to me to not pay another agent the same commission that we'd pay someone from our own MLS...an agent from Philadelphia who sold one of my listings got the full co-broke...
Sure life is not "fair" but wow - I would never take advantage of someone like that "just because it was legal." It's not ethical
to me.
What difference does it make if I belong to the same MLS or not?
I've seen some listings in NY or NJ in our MLS, stating that unless you have a NY or NJ license, all sales must be by referral fee from the other agent; that I can understand as it involves licensing in another state...some of those agents will only pay a 25% referral fee, but some will pay a good hefty referral fee that's pretty darn close to the 3% co-broke - why?
Because it just seems like the RIGHT thing to do.

I know I'm naive - but I don't want anyone to ever call me greedy or a shark because I was legally able to do something that seems to me to be
ethically wrong.
Maybe if it ended up causing a lot more work on behalf of the listing agent, I could see that - but really...
I work for a broker who believes in "nice begets nice." It's much better to play fair and be nice with others, than it is to play hardball and be out only for Number 1.