I think it's not that we don't synthesize enough ACh, but we just get the optimal benefits from infusing nicotine by smoking. Even people who weren't smokers and tried it out and became accustomed to it, know all the benefits for concentration and general enhanced cognitive function. Among those who didn't start smoking naturally at a young age, I would imagine that there are quite a few who don't fit the genetic profile mentioned by the C's and Laura several times over the years, but they still experience benefits, just maybe not as much as those with the genetic profile. As far as I know, it's not a genetic glitch type of thing where the normal Acetyl-choline activity is impaired, just that nicotine is essential and most beneficial to these types with this genetic profile. Also, as mentioned before there can certainly be other substances in tobacco working synergistically with nicotine.
Also, there's more to the myriad benefits of smoking tobacco than just the mimicking of ACh for the nicotinic Acetyl-Choline receptors (that's just ONE great benefit). There's also protection from radioactive particles and other detrimental substances to the respitory system, protection from infections, and much more. You can search on SOTT and the forum, as there's a huge amount of info available about the benefits of smoking tobacco. There's actually quite a lot of scientific evidence that's just not promoted in the mainstream media (similar to the benefits of a grain-free, dairy-free (except butter and ghee if tolerated) ketogenic diet).
Thanks Seekin Truth. It is interesting,and I have read alot about it here and other scientific sources, still smoking is not something I am feeling I want to do, however "having nicotine on hand" is not a bad idea.