I do not think companies use propriety headphone connectors because they save space. A mini 3.5mm jack is tiny. 
Heck my Sony Ericsson mobile phone's fast system port connector is huge, intrusive, badly fastened and falls out. 

The reason I presume is they can charge you profit for extra headsfree sets which would require a special connector to route mic data. They don't include them because they want to rip you off. Sony walkman phone I have did provide a handsfree with media controls (being a walkman phone) but they break too easily.

W is just making a mountain out of a mole hill. As a HTC customer (I have a HTC Touch), I know that there are adapters for my phone (and the T-mobile G1/HTC Dream) that convert the ext port to a mini-usb + headphone jack for around 11quid from expansys.
He also forgets that Nokia, Sony-Erricsson and Motorola have been doing the same thing for years.

W., they do include a dongle. The HTC touch and Diamond both use this USB-to-mini jack 3-in-1 contraption, and may also include another 2-in-1 jack. This is likely overseen as this is a view based on a New York showing and not a package sent for use or review.

The included buds are USB, but standard headphones can in fact be used via an adapter reviewers seem to not be getting.

HTC has THE largest assortment of "stuff" in their packaging I've seen in a phone.

If companies want to use propriety or non-standard headphone connections for space reasons why don't they throw a converter cable/plug in the package, it would cost them very little but at least would show some understanding and respect towards the customer.
It all seems so simple, for cases where companies don't use propriety connections as a way to force people to use their branded stuff that is.