The only people that I know that use email clients on their computers are Linux geeks and employees that are required to use Outlook. Even the majority of the hardcore folks that I know once managed their own email servers for personal uses have long since abandoned them for Gmail.
Email is a huge pile of shit on many levels and while everybody still uses it nobody wants to deal with it.
Maybe if it tied itself into DNSSEC and made it so people actually could trust the originator of the email to be who they claim to be it may move back into the 'things that don't totally suck' category of software, but I am pretty sure it is just too far gone at this point.
Even professionally managed webmail is getting more and more useless. Which is one of the reasons we see the rise of 'social networking'.