Mechanical shutters are not needed, but the choice of designing a camera with an electronic versus a mechanical shutter is up to a companies marketing and engineering teams.
For example, video cameras all employ electronic shutters. The reason being the nature of the images that the camera captures, video. Many specialty high speed and ultra high speed cameras use electronic shutters since a mechanical shutter can not respond fast enough. Cameras with live view or lacking a viewfinder requiring framing through the LCD screen use an electronic shutter or if they also have a mechanical shutter, it's kept open and only actuated during the actual exposure.
A feature of the Foveon imager employed by the SD14 but never implemented in the design by Sigma is the ability to capture video:
Fx17-78-F13D Product Brief (pdf)
So, think of the SD14 as having both an electronic shutter in the imager and a mechanical shutter.
Another aspect of relying solely on an electronic shutter is the imager is always being exposed to the light captured by the lens. If the Sun is within the field of view, then all that light is being focused onto very sensitive electronic elements. Have you ever used an magnifying glass to burn a leaf or piece of paper ? With in seconds, you get a nice plume of smoke. Just imagine that happening in your SD14 ? On another photography web site, I once read the panicked post from a photographer whose dSLR had literally gone up in smoke from Sun damage. I do not recall the model camera so I can not say what shutter design it used.
I remember watching the Moon landing coverage when an astronaut inadvertently aimed the live transmission video camera at the Sun. It lasted only a moment, but the camera was destroyed and useless. The remaining exploration of the Moon of that mission had no live video.
This does not mean that a mechanical shutter eliminates the risk of Sun damage, whether constructed of metal or plastic, focused Sun light can deform and damage a shutter, especially when the shutter is at the focal plane as in the SD14.
Another factor on choosing a shutter design is the philosophy behind the features and performance. If you ever visited Sigma's SD14 web site, they describe the SD14 as putting photographic control back into the hands of the photographer - as what was originally available in analog film based photography. I'm sure this contributed to their choice of employing a mechanical shutter.
