how is a reamer going to cut a chamber that is not true
If that were the case, then why do people pay lots of money for top gunsmiths to replace barrels? Reamers can and do wander, then the lathe could be past its best and have wobbly bearings, the bore may not be set directly in line with the axis of the lathe... Blunt reamer, reamer not cutting on all flutes, chatter... The list goes on...
The same applies to case wall thickness: it simply isn't perfectly uniform, one side will expand more than another, so the result is a case that conforms perfectly to the chamber, but that chamber may not be round and the case head may not be concentric with the rest of the case.
It all depends on the quality of your rifle / chamber / brass: such eccentricities may be tiny and of little consequence. If you don't have a v block and a dial gauge, you may never know.
Yes, sticky extraction is a sign of high pressure. I know of people who have routinely used a mallet to open their bolts (to my mind, they're nuts

). It just goes to show: it IS possible for a case to reach a point where contraction due to elasticity is not enough and the case DOES become larger than the chamber. Although modern actions are very solid, they do still have some give and the bolt head will move back on recoil allowing increase of headspace. Admittedly it won't be as bad as a rear locking Enfield action.
Anyway, my point is this: if you only ever neck size, you will almost certainly reach a point where chambering is not as easy as you started. This depends on how stiff your loads are and the age / history of your brass. If you don't load often, you it may never happen for you.
One top Match Rifle shooter belonging to another email list (of which I am a member) claims that he's always only ever neck sized and never had a problem, but he was definitely in a minority. (Match Rifle is a minority discipline which involves shooting the 308 win cartridge at distances 1000-1200 yards)
I have personally observed that expansion of brass was not uniform in the same box, some rounds chambered easily, others didn't. Then velocity appeared to drop off with increased case volume.
I think the neck-sizing vs full length sizing debate is split:
There are advocates for both. Neck sizing is certainly easier and perhaps in the past, people went on about rounds being fire formed to your chamber, but others disagree.
My other reason for full length sizing, is that I use a Wilson hand die for seating: it makes the Redding competition seater look VERY loose. Even with my custom built rifle with the tightest chamber you've ever seen: fired rounds stick in the seater, neck sizing just isn't an option.
For me, the main problem with full length sizing used to be the lube: I tried a lube pad, spray lube, spray lube using a special rack... Too much lube would cause shoulder dents, not enough and pull out of the sizing die was difficult. Add to that, the distance to the shoulder varied according to amount of lube. In the end, I discovered that part of the problem was Lyman dies: rough as you like. Next problem was the RCBS press: it was wonky and the shell holder didn't line up with the die. The Lyman spray lube I tried also didn't lubricate very well and I always managed to spray too much on. I now use Imperial Sizing die wax (made by Redding) and it is perfect: rub a tiny amount between your fingers then rub a small trace onto the case and that's all you need.