Agents accept work by post or by email in order to make it easier for the writers whose work they've requested. It's not a question of their being more impressed if you send your work in by post.
They certainly don't see paper submissions on their desks as "something that they have to dispose of", and suggesting that they do is insulting to agents and patronising to the writers who send in their paper submissions.
Literary agents are people first, agents second. They want to find good writers in their slush piles; they want writers to succeed. Let's not suggest that they're tyrants, out to squish us. Please.
Not suggesting their tyrants, Old Hack, and if I gave that impression, apologies. My point was that trying to decipher which an agent preferred if they list both on their site in submission guidelines is really a crapshoot.
I personally believe it's a case of that particular one doesn't care. They, much like you say, just want a good story and writer.
Others? If they specify email only, they probably are quite serious about it. Same for those that want MS on a ream of paper.