Ok, I'm going through my CCNA and I've gotten to the section on vlans. It talks about using subinterfaces for routing between vlans. My question is why would you use subints as opposed to just having a trunk connection to a layer 3 device? Is there something gained, or is it just a matter of preference?

Because different VLANs are different broadcast domains, so in order to route between them you need one subinterface for each VLAN. Otherwise, there would be no layer 3 routing.

Also, subinterfaces are meant to be a more general-purpose abstraction on layer 3 devices than an Ethernet trunk. Right off the top of my head I remember that subinterfaces are also used on Frame Relay networks to use a certain DLCI, i.e. if you create a subinterface on an Ethernet interface IOS will enable trunking on the port, and if you do it on a serial interface with Frame Relay encapsulation it will be bound to a virtual circuit.