Well they were originally formed as a single issue party - but they are one of the few of those sort of parties that make it to the mainstream.
They now have a broad based manifesto, and self-evidently offer a lot of Scots something distinctive that they do not get from Labour or the LibDems - I won't mention the Tories, since they have no presence to speak of in Scotland.
Were they to lose the referendum, they would still be a legitimate party, and I doubt that their desire for home rule - they will, I am sure, regroup and try again.
Formed in 1934, Finally negotiated a referendum in 2014 - 80 years of try try and try again... :)