Jonathan Hedley has a great article on why mouse sensors are placed in the middle of the mouse. In brief:
- The best user experience was found to result from placing the mouse ball at the front of the mouse between the fore-finger and thumb.
- Nonetheless, mouse manufacturers placed mouse balls in the center of the mouse in order to achieve better manufacturing efficiency.
- The advent of laser diodes negated the efficiency argument, but sensors are still generally placed in the center of the mouse.

Basically, the position of mouse sensors is a vestige of previous practice - subsequent interaction designers have neglected to correct the established norm.
In the case of mouse sensors, designers are clearly to blame. However some vestiges exist as a result of the momentum of old designs.
The best example of this is the Dvorak keyboard layout:

Dvorak Keybard by Mark Wubben via Flickr
The QWERTY layout is at best an accident, and at worst intentionally designed to place popular keys at a distance from each other to limit typewriter jam. Dvorak on the other hand was intentionally designed for speed, efficiency, comfort and ease of learning.
The superiority of Dvorak notwithstanding, QWERTY is clearly the victor - it is a design that has become so entrenched in its usage that the overwhelming majority of keyboard users are not even aware of an alternative.
It may well be too onerous for the average keyboard user to make the switch, but those who are or employ professional typists, or those who design user interfaces for new devices, should be aware that QWERTY is a design vestige that has been reconsidered. Designing a keyboard interface for a mobile device? Stop to think if the QWERTY keyboard (or any other) is really going to provide the best user experience.
Obviously there’s an overhead involved in continually considering whether the practices you perpetuate are still (or ever were) ideal. But it does pay to occasionally step back and consider why you used a particular design habit, and whether it could be improved either for the job at hand or more broadly.
Say you’re concerned with web user experience. Consider:
- Do you really need users to specify unique usernames? Would an email address do? Perhaps you can provide full functionality without collecting any details at all.
- Did you consider why exactly you need a splash page, or is this just something you do?
- Why include a blogroll down the side of your blog? With so many non-semantic links, is it really useful to your users? Or do you only include one because everybody else does?
- Archives may have been useful for print newspapers, but do you think you need one for your website? Perhaps just make it easier to search by date.
- Do you have anything useful in your header/footer/sidebars? Maybe you could ditch one of these and give your content more prominence.
Obviously, creating a list of possible revisions is not the solution, but these should help to get you started.
Ultimately, whether you’re designing mice or keyboards, websites or houses, you should consider whether you can justify following a particular design practice, or whether it’s merely a vestige of a time that’s passed.
Happy redesigning!