I'm of exactly the same thought as Dch48, I used to use OE and to be honest it was excellent (but effectively no longer supported). I had one inbox and many filters, so I can organise my email based on who it was for (email account) or from, which was generally the content, e.g. newsletters, etc. They override other filters and once set up it is a walk in the park.
When I moved to thunderbird on my desktop system, that was how I set it up having imported my OE emails. However, when I installed it from scratch on my win7 netbook at first it was a right royal pain in the rear wanting to create an inbox for every account even for pop3 accounts. Anyway I on my netbook I don't have it delete mail from the server upon collection, so I can download it on my main desktop system.
I never really figured out how to stop it from creating a new area for each account, but you can also select View, Folders, Unified.
My main email accounts are from my domain name so I never have to switch if I change ISP. I never liked gmails T&C and have never been a fan of webmail (HotnotMail killed that for me). I like to have my email under my control for all aspects including SPAM and Virus scanning and I regularly backup my emails to prevent total loss.