I met one dealer group a year or so ago and they had employed a salesperson purely to deal with email enquiries. They were having it away.
I think dealers forget that people email for a reason. The last thing these people want is a phone call as a reply (unless they've stated that). It's more likely they've emailed because they're sat at their desk at work and are browsing your site when they shouldn't be - hence an email reply makes them look like they're working.
Or they're sat in front of the TV in the evening and hoping to get the information back that they want so they can read it at their leisure.
It's like people who reply to text messages with a phone call. Ask any teenager and they'll tell you that's their pet hate. It's actually one of mine. People type stuff out instead of calling for a reason - their reason - so it's best to reply in the same manner.
And those that don't bother replying at all? Well, as I said above, chances are they're losing a potential sale. For every dealer that doesn't take email enquiries seriously there's several who do, as our Used Car Award winners will testify...