It comes with summoning sickness anyway, so this text really just says it can't block on the first turn, which probably isn't something you'd plan to waste a land creature on.

Well, you also can't use it for mana; which is a limitation.

You couldn't use it for mana if it came into play untapped because of the summoning sickness... so it really isn't a limitation (barring something like Fervor being on the field.)

You probably don't plan to block with Diregraf Ghoul, either. But that doesn't mean that it never happens. I presume wlframe has another drawback coming, though, and there's a reason for his 'work in progress' note.

It also prevents people being confused about whether summoning sickness applies or not.