Why? Just to be like twitter, but different, with no comprehension of why the Twitter limit exists?
The service won't actually be used for "micro-blogging" unless users are constrained by some limit, but 144 is too damn small. Nobody would ever have picked it if they weren't forced to, and these days they aren't.
What infinitesimal fraction of modern Twitter users in App.net's target market do you imagine use the service via SMS? The limit a legacy of an irrelevant use case.
I'd probably have gone a bit higher than 256 myself (disregarding attachment to powers of two), but it's an improvement.
If you go beyond 140 it is then no longer a microblog post, but rather just an ordinary blog post.
The funny thing is that in languages like Japanese and Chinese, where one "character" contains a lot more info than one Latin letter, Twitter is used in quite a different way.