The one window divided up vs multiple windows argument is a personal preference. In the case of an email client, I actually find it more useful to have all the email related components in one window (certainly the folder list and messages list) otherwise you are continually trying to find windows that have got lost behind any number of others.
A desktop utility which let you "glue" windows together would go some way to resolving this issue - I don't necessarily think it is something which every app should implement itself.
Leaning from others is definitely the way forwards. Take the pick of the best from each.