I was taught that, in a jazz context, "slash chords" take two forms:
1. a chord (usually a triad) over a bass note indicated by a diagonal slash with the chord on top and the bass note on the bottom
2. a polychord (two different chords sounding simultaneously) indicated by a horizontal slash with one chord on top and the other chord on the bottom
In actual practice, most "polychords" are simply a different way of looking at a fairly complex chord. For example, C13(#11) could also be written as D over C7. This practice is particularly common among piano players who can think of the top chord in the right hand and the bottom chord in the left hand for the sake of convenience. Guitarists often have a harder time quickly deciphering these types of polychords for several reasons, not the least of which is that we can only play six notes at a time (provided we're playing a six string guitar). Guitarists usually find the chord-over-bass-note approach more convenient.
The other use (and I suppose, technically more "correct" use) of polychords and chord-over-bass-notes is the juxtaposition of two different tonalities. Of course, this is also a bit of a gray area, since if you put your mind to it, you could probably take just about any polychord and relate it to some scale. Here are a couple examples of seemingly unrelated chords being related diatonically upon closer examination:
F# over C: could be related to the C auxiliary (half whole) diminished scale
C over Ab min: could be related to the C (or E or G#) augmented scale (C, D#, E, G, G#, B). Ab minor is Ab, Cb, Eb. Notice enharmonic equivalents D#-Eb, B-Cb, and G#-Ab.
In the case of the first chord, one could call it C7(b9/#11) instead of F# over C. For a guitarist, the former name may be easier to comprehend quickly. In the case of the second chord, one could perhaps call it Ab min/maj7(add#9/b13). This chord symbol is rather lengthy, and it doesn't immediately evoke a commonly known or played tonality. In this case, it would seem to make more sense to use the polychord notation of C over Ab min, even for non-pianists.
As for usage, the idea of polytonality in general is pretty prominent in late romantic period and early twentieth century classical music (Stravinsky did some pretty cool stuff with polytonality in such pieces as "The Rite of Spring" and "L'Histoire du Soldat.") Some jazz musicians who have made use of polytonality include Herbie Hancock (especially his work with Miles Davis in the early to mid sixties), Keith Jarrett, and Bill Frisell, just to name a few.
I've heard guitarists gripe that polychords are too difficult (or even impossible) to play on guitar, so they don't bother trying. While guitarists can't access the chromatic universe as easily as pianists, here are some ways around this problem:
One example that uses several of these techniques together would be to play Eb over A as follows:
6th string: C# on the 9th fret
5th string: open A
4th string: Bb on the 8th fret
3rd string: Eb on the 8th fret
2nd string: G on the 8th fret
1st string: open E
In this case, I'm taking advantage of the open E string to play the fifth of A up an octave. Note that all three notes of the Eb triad could be barred.
Here's an alternate example using yet another open string:
6th: C#, 9th fret
5th: open A
4th: Eb, 13th fret
3rd: open G
2nd: Bb, 11th fret
1st: open E
This gives you a little different sound than that old clunky drop 3 voicing. The disadvantage of this technique is that, unlike drop 2 or drop 3 and many other systems for voicing chords, you pretty much end up having to memorize many more specific voicings as opposed to one shape that can be easily transposed into eleven other keys. But, as one of my teachers used to say, "Music's hard and tricky."