It's definately worth growing a seed crop. Sure, you won't get much if any smoke out of it but so what, it's only one crop. Doing this ensures you've always got a supply of seeds, saving you lots of money in the long run. It's not just the cost which is a concern for me, it's the waiting. I hate waiting for seeds to arrive, I always think they're going to get lost or something.
And another thing, the future is never certain, for all we know seedbanks could get closed down or something which would make it really hard (impossible for some people) to obtain good seeds.