The mountall man page says "this is a temporary tool until init(8) itself" can do it, basically. Why not just use mount -a? Is there a difference between the two, and if so, which should I use for what?
According to the man page, the ubuntu version of mountall does the following :
Canonical does not provide much information on the reason why they had to build a "temporary tool".
According to mount manual, mount -a "[...] causes all filesystems mentioned in fstab to be mounted[...]".
Anyway, I advise you to use mount -a as it works on most unices.