They probably don't currently have the technology in place to meter during the day, and be unlimited at night. It's an excellent idea to encourage users to schedule big downloads overnight. Maybe they don't pursue it because those big downloads are often TV and movie piracy, which competes with Comcast's cable TV business.
Heavy Internet users should consider getting a business account. This also makes it possible to access your home video library remotely using software like Plex, and to set up an FTP server for your friends. It's expensive, but the added capabilities, higher speed, and lack of caps make it worthwhile to some people.
Of course the definition of heavy Internet use is going to change over time as average people start streaming UHD video, and caps will go up, so unless you're in the top 1% of users like I used to be, I wouldn't worry about it.