I think it's very marginal whether an advantage was gained. As the OP pointed out, nobody else was doing it, which they would if it was an advantage. I think Alonso just preferred that line and felt he gained more from his higher apex speed than he lost accelerating with one rear wheel on the astroturf and the other on the serated kerbstones (as compared to the others who had one rear wheel on the tarmac and one on the kerbs under acceleration). If the stewards do nothing about it then Alonso is bound to take whatever line he prefers, including leaving the track.
I've said it before and I'll say it again, it is not permitted to intentionally leave the track without justifiable reason, and you can be penalised for doing so whether an advantage is gained or not.
In my view if a driver is leaving the track every lap, like Webber was at Silverstone and Spa, for example, then it cannot be deemed accidental or justifiable so it should be penalised whether there is advantage or not. The driver should be warned and then if he fails to heed the warnings he should be given a drive-through. If he does it in practice or qualifying the relevant laptimes should be deleted.
There needs to be some consistency of approach from the stewards about this. They should deploy spotters to the parts of the track where an advantage can potentially be gained by corner cutting or running wide (nearly every corner at some newer tracks, alas), so they know exactly how many times each driver has left the track at a particular corner. Each instance of going off track should be judged by the spotter to fall into one of three categories:
1. An obvious mistake with time loss, or justifiably going off track in avoidance or being forced off by another car.
2. Deliberate off-track driving with clear advantage, such going off track to make a pass or gaining a clear speed advantage.
3. Apparently intentional driving off-track without clear advantage or disadvantage.
Category 1 should be ignored. Category 2 should be reported to race control for investigation to see if further action is needed. Category 3 should be totted up and, when a particular car has three strikes, he should be warned on the timing screens (like they do in ACO-rules racing: in this case "Car 3 must respect track limits at Turn 6"), and then if the driver doesn't mend his ways, a further two strikes should result in an automatic drive-through.