The problem then becomes, "Man, that guy was a total tool and I should report him, but I'd rather get right back into the game/do something else rather than take five minutes to fill out a report."
Sure, you get rid of spam reports, but you also get rid of a lot of the worthwhile ones.
Someone leaves my game and I have to endure a hellish 20 minute downward spiral until surrender. When the game's over, I'm not going to want to have to then open up my browser or a new tab, load up the LoL website, log into the support page, and then file a report.
Oh so sorry if you actually have to sacrifice your precious time to report. -.-