Pretalks are important in the sense that if you screw up the pretalk, you can screw up your chances, but not in the sense that skipping it is bad.
The pretalk serves two main purposes - to make sure that the subject understands what is asked of them and to make sure that they are willing to do it. That's it.
If you're using the pretalk to give your subject a lecture on the theory of hypnosis and trying to give them a verbal level understanding of what to do, then you're doing it wrong. All hypnosis is not self hypnosis. They don't need to know how to lead - they just have to know how to follow. And for that, they just have to have an idea of how it would feel. It's not about having an understanding in semantic memory, its about having an understanding in implicit memory. If you say "Jump!" and they ask "How high?", you're off to a bad start. You want them to just jump. If the height is important, it's your job as the hypnotist to specify that.
If someone is consenting to having you hypnotize them, then you presumably have something that they want. So just do it. The only time you run into a problem with this is when they think they want to try it, but haven't actually committed to a decision yet. If they congruently say that they want to be hypnotized, then you're set. No pretalk is necessary to make sure that they are willing.
So the alternative is to skip the pretalk altogether and just wing it. In fact, I much prefer this approach. If you're well calibrated, you can foresee which suggestions won't be understood or complied to and preemptively work around that - like a distributed pretalk. And when it doesn't go perfectly, you can always debug it in real time. Sometimes people will use pretalks to warn against some expected difficulties, but there is no sense in worrying about issues that haven't come up yet.
There is another use of pretalks, which is actually pretty cool - as a vehicle for conversationally seeding suggestions. You can just give the subject a pretalk to tell them what you're going to suggest to them and how they're going to respond. If they're high in expectancy already, the pretalk can prime them for the suggestions. Then you do the official "hypnosis" ritual - tell them and have them respond. The downside is that they're not full blown tranced out during the pretalk, so you'll have to dance more carefully around potential objections or shoot for conversational trance.
Although pretalks can be useful in certain cases, they are by no means necessary for successful hypnosis.