If you visit the "directory of blogs" page, you'll see a full selection of the filters: http://www.submithub.com/blogs
Here's what I like about the very small flat fee:
1. It is small enough for even modestly successful musicians to cover it. It isn't a monetary barrier to entry.
2. It is likely large enough to prevent the shotgun approach. This is one of the bigger problems of finding new talent for record labels and music writers, especially in the digital age when emailing or messaging someone a link to your song approaches zero cost...you can't realistically listen to every unsolicited track, so you have to filter somehow. This, at least, insures you don't get a bunch of stuff in genres you don't care about or know anything about. People have to choose a little more carefully.
3. It provides an incentive for writers and musicians to form a relationship so they'll work together again (with the blogger getting tipped again and the artist getting more coverage or useful feedback); it starts out as a monetary transaction, but once they've connected up and like each other, they could work toward building a scene (and the network that comes with that). In music, a scene is a force multiplier for everyone involved (e.g., Seattle grunge, Madchester, Atlanta dirty south).
Increasing the fee for big sites looks more like payola than merely a filtering tool to determine who has gone to the trouble to figure out the best handful of writers or labels to reach out to. If the profit of the site begins to come from the musicians they're reviewing, their incentives start to get lopsided in the wrong direction. I'm not saying all bloggers would do it, but there would be incentive to boost stats (with clickbaity stuff), and to get as many submissions as possible. Incentives should, IMHO, be directed toward increasing quality, maximizing the time a writer can spend on the artists they choose to cover, and providing the best feedback for the artists.
Anything that gives incentive to cheat the system will break it and reduce the quality of the network...likely even killing it. If there's even a slight notion that people are scamming musicians with this, it will (maybe deservedly) die.