Virtual appliances - great for feature evaluation, but watch out for performance!
As a user, I think that in general, the more exposure to "test" virtual appliances, the better. That said, I can understand why suppliers aren't keen - we use Isilon, and the physical kit/performance is great - but their virtual appliances can be *very* slow, unless you're running it on a hypervisor with access to SSDs. Run a virtual cluster on a bog-standard laptop with a 5k4 disks, and performance will not be great!
If I were a supplier, I would be worried that somebody just playing with a virtual appliance, without some initial hand-holding and qualification of hypervisor limitations, would get the wrong impression about the performance of the app.