Being an "international alternate" doesn't mean that anyone is going to actually go there. It just means that the airlines can carry less fuel on the Perth flights if the weather requires an alternate. And as Busselton does not have any precision approach aids, the weather there will have to be pretty good.
This airport has been a bit of a local saga for a while, and they've been desperately trying to get an airline to go there, whilst at the same time becoming very noise sensitive, and tossing out the flying schools. This Jetstar service is supposedly heavily subsidised, so I guess it will be interesting to see if whomever is providing the subsidy is prepared to keep it up, and if the service is viable.