I've heard the "add new cards" argument before, and honestly, I think that it makes the rules for what is legal too complex. Modern has the most complicated rules for what is and isn't legal. Standard is "the last two fall expansions and any accompanying expansions and Core Sets." Legacy is "everything not specifically banned." Modern is "everything that has ever been Standard legal, starting with 8th Edition." It is not "everything in the new card frame," because there are cards that have never been printed with the new frame that are Modern legal (ex. Lord of Atlantis) and cards that have only ever been printed with the new frame, sometimes in multiple printings, but are not Modern legal (ex. Maelstrom Wanderer).

When we start letting in some cards that have never been in Standard, but not most, it makes things very convoluted. I assume that anyone posting in this thread is reasonably enfranchised as a player (can't imagine getting too interested in MM2015 spoilers otherwise). Imagine what it's like for a player who's new to Modern, three sets past the "we will let brand new cards in specific expansions enter Modern without passing through Standard." How do you sort through the morass of sets that were being printed ten years before you started playing, and also navigate which of the new products are legal and which aren't? Hell, I once saw a guy brewing with [[Dismiss]] on /r/ModernMagic because he saw the Commander 2013 copy and thought it was Modern legal.

This is stuff that seems really simple to enfranchised players like us, but can be mind-boggling to people just getting in, or just getting back in. For reference, I didn't play between Eventide and Return to Ravnica. It took me a good three months to memorize which colors "Bant," "Jund," "Naya," "Grixis," and "Esper" meant. This just gets too complicated, too quickly.