Why is power consumption sometimes given in mA and not in units of Watts?
Most electronic systems use a fixed voltage and thus you can determine the power based off of the current. This can also come up in systems that use linear regulators in which your current stays the same regardless of the voltage you apply (within a certain range of course). For these systems it makes tons more sense to give a current rating since the power in watts will change based off of the voltage you apply to it.

Also many times it is the current that is the limiting factor because of things like trace/wire width and not necessarily the power that is being consumed.

USB is an example of this limit.  The USB specification limits the current used by bus powered devices to a total value per port.  The limit is 500mA for v2.0, and 900mA for v3.0; see http://en.wikipedia.org/wiki/Universal_Serial_Bus#Power