First, it would be significantly more of a hassle to boot up a separate OS for the purpose of executing a short-lived task (such a submitting homework, or doing banking, as others have mentioned). Related to that, there is a slight disconnect between the host filesystem and the guest filesystem. The more convenience one has (e.g. greater transparency and sharing) the greater the risk.

Related, but separate from that: how would you know the VM was compromised and thus should be destroyed? One could presumably just periodically destroy (or revert to snapshot). Perhaps even if it was compromised, maybe the short lifespan of the VM would limit the damage to others.