It could shelve demands for proper reform
I don't get the 'hold out for a better electoral reform theory' of voting No. (I assume such people have serious qualms voting at just about every election, presumably not voting a lot, instead preferring to hold out for their dream candidate to register to run in their constituency.)
At any rate, I think the holding out for something better is rather optimistic thinking. Under what circumstances would a No vote help lead to more ambitious electoral reform being on the table within the next generation?
This was the system Labour were ostensibly inclined to adopt - if it's rejected, then they're unlikely to pursue it at any stage (or I'd presume, any change at all).
The Conservatives want no change at all.
This is the best the LibDems could get, and it seems they're unlikely to get another swing at things.