For The Love Of Port

I'm new here and to port so I can ask dumb questions with aplomb (at least for awhile, I suppose).

I assume natural corks are superior to synthetic corks -- because the best wines of all types pay a premium to use quality natural corks -- but what advantage do they have? They have the disadvantage of occasionally damaging the wine as when the wine is "corked" and of being more expensive. What makes them superior?

Additionally, why not screw caps (gasp!)? I assume screw caps, besides being ugly, would prevent wine from evolving in the bottle or evolving as desired in the bottle. Is this the case?

I have always assumed that there is some sort of slow exchange between the outside air and the inside wine that occurs across the natural cork, but on thinking about this further recently I've begun to question this assumption. If such an exchange were to happen as I envisioned it -- an 'exchange' not unlike what takes place in a wood cask -- wouldn't the level of the port in the bottle decrease? I know sometimes the level of the wine goes down -- ullage -- but this isn't an expected effect of optimal long term storage but rather an undesirable and faulty effect. That is, wine can and preferrably DOES mature well without ullage.

A further related question. When port does get lost -- evaporating through the wood (for example long term aging of tawnies) and/or across a cork -- does the alcohol selectively evaporate first, because it has a lower evaporative temperature?

I know there are several questions here and that they aren't particularly well articulated. But they seem to be related. Can anyone shed any light on this subject for me? This came up as a question when I was talking about natural corks vs synthetic corks vs screw caps and I realized my assumptions were pretty arbitrary and unsupported by any solid information or knowledge.

Some topics!
Lots of ink spilled on them. Lots of hystrionics, hyperbol, rational and irrational discussion. Maybe not in this forum, but certainly others.
I think universally, the thinking is that plastic corks do not fuction well. Certainly not for long term storage where premature oxidation occured. They did not live up to their inital promise.
The current argument is natural cork versus screw top caps. Newer screw tops are very uniform. The liners on them can be adjusted to allow a well controlled amount of oxidation of the bottle contents. I think the majority of wine chemists agree that aging does not require oxygen, however some byproducts of wine production and fermentation require oxygen to prevent the formation of unwanted compounds in the bottle. So a little oxygen can sometimes be a good thing. Corks are traditional but may be tainted. In addition the oxygen ingress allowed by corks vary hugely. Not very predicatable, hence one source of bottle variation. Nothing beats a good cork, however not every cork is good, even the very expensive ones. This is the dilemma in the sealing of wine bottles.