Heavy users are the ones who generate the content that gives the site value in the first place, though. Advertisers aren't there to sell products to the 1% of users who make most of the posts, they're there to sell to the 99% reading those posts. Making posting on twitter a bigger pain for the people who do most of the posting—and especially for celebrity and brand accounts that get tons of "engagement" they want/need to keep track of—is probably not a great move.