You are also right about the fact Google might even be clever with the services that do not implement 301, however there are other factors involved as well, which others better explain.
We could use it to shorten our own Tweets I suppose...
A) If any URL uses a 301 Redirect (permanently moved HTTP header) Google will often pass on the juicy page rank to the new URL, whether it's a Short URL service or regular URL.
B) Even if a Short URL service doesn't correctly 301 Redirect, I'm sure Google factor that in, and probably pass on the link juice anyway in some cases. Considering the way we all know how Short URL services are used, Google probably knows too, and factors that in to their algorithms - I know I would if I were on the Googlebot team.
Considering the vast amount of Twitter users, I'd say they help quite a few people, otherwise they just wouldn't be used. And, to be fair, with a service like Twitter, there isn't much of an alternative.
It's certainly true that if you use a Short URL service, you're stuck if they go offline (and many have done), which is why I think it would benefit Freeola to use freeo.la for any Tweets (and to allow Freeola customers to use it for the same purpose), as you'd then be in control of your own URLs.
Machie, I only noticed they'd registered it by accident, as for some reason, I just decided to type it in to the address bar, and was redirected to freeola.com - and eeo.la is available!
I see it as more of a fad, and since bit.ly became the championed short URL service, one by one the others close their sites down, breaking all the links that used to link to your content.