Dear Miss Snark:
We've all heard the daunting statistics. Agents receive anywhere from 5,000 to 30,000 queries a year. Some have never taken on a client from the slushpile. Others say they've taken on maybe 4 or 5 over the years. Best case seems to be 2 or 3 a year from the slush. They ALL complain there's simply no time to read and respond to everything sent them.
So why do agents even bother reading the slush? From a business perspective, the return on investment doesn't seem to justify the query process. Why does the practice exist, and why does it continue when agents could easily solicit manuscripts from other avenues such as conferences, contests, organizations, etc.?
I'm not sure what the source of your statistics is but they do not jibe with mine. Fully half my clients came over the transom (not literally, although there was that one girl...) and when I started up, more like 85%.
One of the smartest most successful agents I have had the privilege of guzzling sake with says "there's gold in that there pile" and refuses to even use the word "slush". I look at her sales record and I pay attention to what she says.
Every time I talk to Kristin Nelson, another very smart VERY savvy agent, she talks about how she reads her submissions and finds people there.
I question your data and thus your conclusion.
I've found more clients via the transom then I have from contests, and conferences.
The only source that comes close to the transom is referrals. Publishers or editors or clients who gave my name to people who write well provided me about half my client list right now.
Of the last five books I sold, three were from transom clients, two from referral.