I have a sneaking suspicion that the ability to connect tweets to get something resembling a longer tweet is probably considered to be a bit of a cop out.

The reasoning behind retaining the 140 character limit seems to be that if it's longer than 140 characters - post it elsewhere, then link to it if you want it on your feed. Trying to cater to the market where you can create messages to your social network that are somewhat untargeted seems like it would make twitter less able to offer the 'of the moment brevity' they want, and really would result in them being a partially implemented feature that already exists on other social media platforms.