I have the same question and problem. I think there are two paths to solution. In both cases, I’m suposing you’re using Mailchimp syncronization extension, right?
1. Registering in Magento trough the New customer form, extending the number of fields (which is not easy) and creating the same fields in Mailchimp. The problem here is creating those new fields in the customer object at Magento and sending the information you collect at registration or new customer form to Mailchimp.
2. That each new customer registers themselfs trough Mailchimp and basic info (the same considered by Magento) passes to Magento. The problem here is that Mailchimp sync extension works only from Magento to Mailchimp, and not in the other direction.
In both cases, you would have all the information you need for segmentation at Mailchimp, which is great because you can send personalized mail campaigns from there… but, we have those issues reported by me above.
I have the feeling that there is a lack of functionalities which help to collect as much info from customer (basic info as demografic, geografic, preferences and orders history), to process it for segmentation and find new info (like Business inteligence) and finally sending accurate messages to each of them, hopefully trough Mailchimp.
Having that would be awesome! Is there something we have not discover yet?
Thanks everybody who could give some advice.