I believe that pauluk gave the primary reasons for grounding the positive terminal on a DC system.
With respect to electronic equipment on DC supplies, I would like to expand a bit. Clear the supply to the devices requires both positive and negative, but especially with respect to 'three terminal devices' (vacuum tubes, transistors, etc) you see a useful asymmetry: The control signal to the device (gate drive, base drive, etc) is referenced to one of the terminals. If you ground that terminal, then the voltages associated with the gate drive are greatly reduced. For example, an N channel MOSFET is controlled by a gate voltage relative to the source terminal of a few volts, even for MOSFETS (or IGBTs) rated for drain to source voltages of thousands of volts. In circuits that use these devices, it generally makes sense to ground the source terminal. (Note that many interesting circuits use MOSFETS in ways where at least some devices can not be grounded.) Some devices connect the source lead to the metal body of the device; if the source lead is at ground potential, then you can ground your heat sinks, a very attractive design feature
I believe that you left out one additional sort of grounding used in DC systems, at least in small signal circuits, and in large VSD DC links: neutral grounding, where you have +Ve and -Ve, and ground '0' in between the two. Common power supplies for things like op-amps are +-15V, and in most VSD systems you have a 'ungrounded' DC bus supplied by rectifying the supply AC...but the supply system is grounded, so the net result is a DC bus which is balanced about ground potential.