A big disadvantage I have found is when buying something from a site I have visited in the past. I have totally forgetten the login id that I chose and it refuses to set up a new one as the email address is already registered and offers to send a password reminder if I enter the (forgotten) userid.
On the other hand for sites I buy from regularly it is great to have not only contact details but also special delivery instructions remembered.
On the whole I would be in favour considering the number of problems that I have seen from missing confirmation emails in the past. But under the following conditions:
1. Use email address as the id.
2. Do not require login before filling the basket.
3. Prompt to enter password or create account at checkout while capturing email address.
4. Have full tracking on line so that customer can view the status of the order through to despatch.
5. Explain clearly that you are asking for login so that they can view the progress of their order at any time of day or night.