That only works though, ich, if the UK government responds in the right way -- which, to my mind, is refusing the offer *now* (ok, in two years) but guaranteeing one in the not-too-distant future after that. If they call Sturgeon's bluff one way or another I don't see the outcome being very pretty. It stands to reason that committing to Brexit means trying to get the best out of it possible, and that should be a lot easier if the UK is actually U when dealing with the EU, now and for as long as the negotiations take.
Alternatively, refusing the vote smacks of contempt, probably makes the divides that allow the SNP to keep calling for a referendum even stronger, and could make the break-up effectively inevitable.
Finally, it's odd timing in one sense because I thought Sturgeon was waiting for a marked and clear swing in favour of independence in polls, but this swing has not yet materialised. Maybe it will as Brexit starts and continues, but if it never does then it's a sure way to guarantee that the SNP's ambitions will die for a long time yet. Two defeats in five years? How can the party, let along Sturgeon, recover from that?
This announcement only makes sense as an attempt to shape the negotiations of Brexit, rather than an attempt to seek independence. Since it's happened before the Article 50 Bill has passed and before Article 50 has been triggered, I almost wonder if it's a last desperate attempt to frighten MPs into halting the process at the last second, or at least to keep the Lords amendments.
I think it's probably true that the UK government has so far shown signs of listening to and then ignoring anyone who doesn't agree with their intended negotiating pitch, which looks to be "Brexit Max" or hard Brexit or whatever. So perhaps also it's an attempt to just force May to listen to the Scots government (ie, to Sturgeon). Which would be nice, but this is playing with fire.
No, as I see it this is premature, and therefore very reckless. Could work spectacularly, could fail spectacularly, I don't think it will fizzle into irrelevance though.