You get the users you design for (Score 1)

As a UI designer by trade I always like to throw things like the iPod, Google, Firefox, World of Warcraft and Nintendo Wii into any argument about the validity of designing for your users. These products success against their competitors speak volumes about the impact of a good user experience (the user experience is a vastly more complex beast than just the UI, but this is what usability is all about - designing the environment to try and improve the user experience).

Yup these are mostly commercial products, probably because commercial products have a vested interest in continued use by their user base as well as growing their user base. Not to mention usually having more funding to be able to pay for user experience professionals.

As has already been mentioned numerous times, most open source/free software is written by the developer for the developer. This is fine as long as the people working on the project are happy with a niche user base. If you are developing something for yourself and making it available to others then more power to you.

There is always exceptions to the rule usually where need or cost outweighs usability.

If, however, your are looking to grow your project and see widespread adoption/interest, you are much more likely to see this happen if your interface appeals to a wider audience. Of course you shouldn't design for 'everyone' but you target the user types that you want your software to appeal to.