Currently, with TurboActivate 3.x you need to use product keys for verified trials. This means they (a) use up activations for your account (which is less than ideal), and they're harder to manage. Yes, you can delete verified trial product keys, but that means you have to separately manage who has already gotten a trial.
With TurboActivate 4.0 (out soon) we've added product-key-less verified trials. The verified trial limit is separate from the activation limit (and much higher -- because more users try rather than buy), and this new system inherently tracks who used verified trials (so you don't have to). More information will be posted when TA 4.0 is released.