Meaning of Q and I in all-software QPSK communication systems? Q and I seem to come up a lot in discussions of digital communications, such as the comprehensive answer provided to this question:

FFT window phase syncronising

It seems like the Q and I channels ultimately are combined into a sin wave of a given frequency, phase and amplitude.  If the communications system is all software (for example in a software-defined radio) is it necessary to worry about "Q" and "I"?  Is this just an artifact of when receivers and transmitters were electronics based?

To make the question more concrete, the link provided talks about putting the even bits onto Q and the odd bits onto I.  If this is the same thing as creating a sin wave of phase 0, 90, 180, or 270, then why bother with this seemingly unnecessary notation?  Perhaps the pulse shaping (like an RRC filter) needs to be independently applied to the Q and I channels?
A point in 2D space can be represented by either X and Y coordinates, or a distance and angle (relative to some reference point and axis).  However, the point measurement errors or noise is some common real systems is more likely to be uniform in the the units of an X,Y coordinate system, which may make error analysis and numerical error control in this space more tractable, thus lowering the computational requirement for most types of processing, even in purely digital systems.