Something that struck me about these is that they are for twitter but still relatively long url's in themselves. longreply.com is 13 characters, twitlonger.com is 14, and the last mentioned in this discussion logilogi.org is 12. For me, that alone makes these not worth a second look.
I realize that if you're using this, you likely have room under 140 to post the URL, but I believe that in practice many will use this to continue posts -- I'd still post the beginning and then finish in one of these pastebins, rather than just post the URL. People are not going to just click on a rogue URL but they are more likely with a bit of an intro.
Seriously. I have used/seen the exact same functionality from other services for at least a year, and they give you a shorter url to boot. What's new about this? Sparseness?
I have to agree, if you've a throwaway comment you want to make that's too long for twitter, then I can't imagine anyone's that interested anyway. Learn to shorten yourself, or make a comment you don't mind being a little more permanent on a service like posterous (that then posts to twitter in an identical way to these "paste bins", but with hashtags and a shorter url (post.ly))