Hey Dave I'm going to have to disagree with your disagreement. I get a bit frustrated with the WWAD (what would amazon do) approach to what an optimised experience should look like. The reason is that Amazon have a different playing field to 95% of us and that's all to do with branding.
They already have awareness, trust, validation, history -- all the things that lower customers leaps of faith.
This means the balancing act between friction and business requirements is on a different scale - and a different planet. Essentially they can do things that ask a user for more, because they're Amazon.
Same goes for Apple... they can launch a campaign that simply says ' this changes everything' and people will listen. If I did that, people would say' what the hell are you talking about'.
So looking specifically at account logins you need to make a call.
Your benefits are to be able to both create a more seamless checkout experience the second time round for the customer, and give you remarking opportunities.
You cost, is your going to lose some first purchases sales because of the additional friction.
If in your case (like Amazon) the benefit you get outweighs the cost then go for it. If it doesn't then you might want to go without.
Some will test it, others will go on gut feel.