Glut, without a good review system, causes eventual market bubble-bursting and cynicism over the general quality of 3rd party publishers. 10 for 10 great products is better than 15 of 50 great products, because the people who buy the 35 terrible ones become mistrustful of all of them and some stop buying altogether. It's good for a short term, but harms the market overall in the long term.
The only good counter to this is a solid system of user ratings. This did not really exist for the 3e glut - it was pretty random and word of mouth only did so much if you had to search out product reviews. EW had a great review system - but you had to go look for the product, and you couldn't buy it through EW.
On the other hand, things like Google Marketplace do provide a pretty solid system for user ratings. If the suspicion we keep hearing bandied about turns out to be correct, and WOTC is creating such a marketplace online, then that may well solve for the glut issue while still allowing for a glut of products to be produced.