TLDR: everything above is true, but guest checkouts = 20% or so increase in sales. Read the famous http://www.uie.com/articles/three_hund_million_button/ to know why.
I normally don't resurrect ancient topics, but I'm looking for something fairly specific and this ranked highly... so I figure I should put forward an alternate viewpoint.
Pretty much everything above is true - guest checkouts ARE NOT good. They do NOT save time (excepting the entry of a password, twice). They ruin data integrity, create duplicate entities and do all sorts of "bad things" to your database.
No argument. Here is the other side of the coin:
From memory when I did the research on this, making "guest checkout" available increased sales by 15-25%. That's right - enabling this piece of evil bumps sales by a full quarter or thereabouts. when implemented on one of my stores, the increase in sales, adjusted for growth was 20-22% guest checkouts are painful to support/manage and there's just no way to make them NOT suck it doesn't have to be "a guest checkout" - it has to be "you can buy stuff without having to 'sign up' with us"
Visitors "want what they want" from a website. They don't know (or care) if it is logical, sane, acceptable technical practice etc. They just want stuff. Visitor resistance to registration has nothing to do with time saving, good data integrity or anything else - guest checkout is a 100% psychological winner. Most visitors are very suspicious (paranoid) that every request "to register" is really a sinister plot to get their super secret details and spam them and market to them. Most visitors have no idea that the only extra bit of info they would supply is a password, which cannot be read by the store owner anyway. No one wants to try and remember "another set of details", but there are also many many stores where they will not use social sign on either. They don't want a relationship - they want to buy your stuff and leave as quickly as possible - you smell bad.
I hated guest checkout - for all the reasons here. I still do. What's wrong with creating an account? Jeeees.
Recently though, I understand. I don't agree with it, but I understand it. They're scared, not good with computers, used to screwing up logins, apprehensive and top of it all - paranoid that they are giving up their privacy - to someone they don't know and do not want to know. Never mind what they post on Facebook or Twitter - your little store is a BIG concern.
I'm not saying YES or NO to guest checkout - what I am saying is that this whole thread is true, but only half the story. The other half of the story is best read here:
http://www.uie.com/articles/three_hund_million_button/
Yep, a $300,000,000 button because people just don't like registration. And it's not at all logical.