It would be a very interesting excercise to compare seizure figures for alternative drugs when examining the effects of supply reduction on users in general. To accurately indicate any shift in user trends towards alternative drugs, this would need to be done at a time when markets have been able to fill the void, i.e. not instantly a shortage commences. If this was compared to user surveys, I'm sure a common pattern would emerge where most users turned to alternatives rather than give up entirely.
If such patterns were shown to have already occurred in Australia, this is surely evidence that the current system should not be the foundation for future policy.
I guess the major argument against this will be; as law enforcement and detection improve, we will one day see an extended period of no availability. Most hardline antidrug supporters must be able to envisage a world which has been stripped completely of illegal drugs for years, a world where people have just forgotten about them or simply can't obtain them and wouldn't dare trying. An absolutely unrealistic view, but not all that uncommon I fear
As BT mentioned, stats can be used for anything. It not only what figures are selected, its also how they are extrapolated and presented.