It’d make for a good anti-spam measure if there was a limit to the number of DMs users could send to other people who don’t follow them back. It’d mean people can still use Twitter DMs like a normal messaging service (which isn’t something I care for, but I know some people use it like that).

As it is, it just feels similar to the whole “rate limiting the number of tweets people can view per day” thing, where they’re taking the most obvious route to reducing bandwidth usage by restricting users.