I love the idea of online submission (me being a wannabee, though, could possibly hinder the effectiveness of this post). I don't think it enables more wannabee writers to submit, but just makes it easier on the writer & the agent to get through the process. The writer is more likely than not looking up the agent online anyway, so they get to skip a step while querying, making it cheaper & convenient for them. The agent gets uniform submissions (mostly. Nobody's stopping the writer from adding unwanted things to those fields or misunderstanding what they're asking for), & can peruse them quickly, & doesn't have to worry about whether a SASE was included & sending back (or tossing) a ms., especially since they won't have one unless they solicit it at that point. More queries, less slush.
What I don't understand are the ones that have fields asking to submit different things than what they request for a written query letter. One I saw had three different questions regarding experience & a spot for a query-letter-type summary, but if you mailed the submission, they wanted the query letter, synopsis, and a few pages of the ms.