The reason that Cubesats are launched from the ISS has more to do with launch availability. It's rather difficult to launch as such a small secondary payload. Furthermore, for small satellites, one has to be close to Earth (To reenter in a reasonable amount of time, and it assists with the communication budget), which is even more difficult to get. Virtually all of the launches that are low enough to have the FCC required 25 year reentry period of time are either to the ISS, or spy satellites. Spy satellites don't like having secondary payloads (Although there have recently been a few, see this link), thus the ISS is one of the best, if not the best, options for a satellite orbit. Note that if one has the required fuel, one could launch as a secondary launch to most Earth observing satellites, which there are a large number.
Okay, so if you are going to be a secondary payload heading to the ISS, why actually launch from the ISS? The reason is that the space around the ISS is carefully controlled. By launching from the ISS, they can do so at a time and in a manner that will ensure that they don't re-collide with such objects. They might choose to do so right before a thruster firing, for instance. It also makes it simpler to launch, as no rocket control is required to separate the payload, and the satellites can remain as any other cargo. Launch systems are complex devices, launching directly to the ISS avoids having one of these systems.
Cubesats are missions of opportunity, and can't afford to choose their launch trajectories very carefully. Bottom line is, they do what they can, and virtually all of them I'm familiar with don't care that much about what their orbit is (Just that they have a high enough inclination for them to control from their ground station). All cubesat missions I'm familiar with have waited years on the ground to launch. Mostly they just want to make it to orbit, they don't care about the inclination of the orbit very much.