My gut reaction to the postal acceptance exception (try saying that three times fast while pissed) is similar to yours. Still, there are a few reasons behind the apparent incoherence. I don't find them compelling. You've raised an important question, and one I happen to find very interesting. Apologies if this isn't particularly clear--only just got up and am trying to write this without getting my notes out and bestrewing the desk with them.
I think it's first worth considering the mooted justifications of the postal acceptance rule.
1. At a pragmatic level, it's necessary to have some clear indication of when an acceptance sent through the post takes effect. (This much is clear, though obviously the need for *a* clear rule tells us little about whether this is the correct one. A number of academics, notably McKendrick, agree with you. I think Beatson in Anson's Law of Contract is a bit more divided.)
2. The argument put forward in Household Fire and Insurance Co. v. Grant (I think, cba to sift through my notes this time of morning--edit, checked wiki, that's the one--bear in mind this is English law, so I don't know if the Scottish precedents are different) that the post office is the agent of both the offeror and the offeree. I find this reasoning strained, at best. But, the general attitude in Household & Grant is that the offeror takes on the risk of postal acceptance unless he expressly includes it. That includes the risk that an acceptance will reach him before the revocation does.
There has been some suggestion that revocation should be permitted, as there was never a shared state of mind from an objective standpoint. There is also some suggestion that revocation should be permitted, so long as it does not permit speculation. The postal acceptance/revocation rules aren't designed to facilitate speculation in currency or commodities, for example. Finally, there's a case distinguishing Household (I'm not absolutely certain on this) which says that the postal acceptance rule shouldn't be applied where it would lead to manifest inconvenience and absurdity. I happen to think the postal acceptance rule poses the risk of manifest inconvenience and absurdity on far too regular a basis, and ought to be scrapped, but it's not as easy to consign to the dustbin as one might hope. There are problems with other approaches, which is one of the reasons why it's been settled law--though distinguished for fax, email, telex communication--for 130 years.
(I
contract, too.)