Being a member I don't like the idea for 2 reasons, first off I want to have the same chance as anyone else to get a shiny because they're meant to be difficult to find, they're special because they're exclusive and only owned by very few people. If all of a sudden I have better odds to catch one they're value and importance is lessened.
Secondly, seeing as a lot of the game's economy is run on trade and pokemon with good IVs, EVs, and Shinies are big ticket items. I could see Members having a very easy time farming up enough money monthly to easily pay for MS Medallion with in game money and also over saturating the market with shinies. The developers would sell less Membership Monthly which is a loss to them and the Shinies in game would no longer be a cool rare exclusive pokemon but just a P2W Glamour.