In my experience, a lot of hosts deny shell access via SSH but allow SFTP access. ie. try to connect via ssh2_exec() and all you'll get back is this:
Shell access is not enabled on your account!
If you need shell access please contact support.
And that's even assuming you get output back at all - all to often, ssh2_exec() returns prematurely, as noted here:
SFTP, in contrast, seems to be something that works much more reliably. That said, the PECL ssh2 extension, itself, isn't often installed, in my experience. I've maintained webpages for a university and a state government agency and, in both cases, the servers could only be connected to via SFTP but the available PHP extensions were rather sparse and the ssh2 PECL extension was not among them. In my experience, actually, those most likely to require the use SFTP are also those least likely to have feature packed servers. For these people, a pure-PHP implementation of SFTP would be nice and phpseclib provides that:
phpseclib is GPL compatible, as well, per this:
As such, including it into Drupal's sources shouldn't be a problem.