Inessential hits on one of the ugly truths about free software (which is separate from open source) - if a given niche in software goes free, you end up with 80% solutions:

There are some email clients I personally like—Mailsmith and mutt, in particular—but I’m not the first person or the last to say that there is no Ultimate Email Client for OS X. Justin is right in that nobody can afford to create it. Even if you made Pretty Much the Greatest Email Client Ever, it would be hard to compete against Mail.app and gmail and so on. Email clients are like air: people don’t want to pay for something so basic. (Okay, some rare people will.) What’s frustrating is the sense that, by the year 2005, we shouldhave a great email client. It’s not like it’s new technology. It could be done. The problem is the economics.