I think looking at it this way is a bit naive. While I agree that "attachments" shouldn't take up tweet text, the 140 character limit is a core component of the service. Call it a micro blog or a mass SMS, Twitter is 140 characters.
That said, I think most comments are missing the point - having attachments not take up characters really won't make the service any more useful to the mainstream... That's the real problem in my opinion.
I don't know, if you post a link as well as an image you're losing 24 characters from a 140 character limit. You lose just less than 20% of the characters available. I think a 20% increase is fairly substantial.
Especially considering most changes are pandering to brands, and this is a use case that we use a lot. (Whether Twitter is a useful channel aside...) extra space for copy will always go down well with the people paying the bills.
That said, I think most comments are missing the point - having attachments not take up characters really won't make the service any more useful to the mainstream... That's the real problem in my opinion.