I play bass and everything I've ever been taught says slash chords are used to designate the bass note. This can either be an inversion or outside the chord, essentially making it a different chord. This does mean that if the bass player is playing the note you don't necessarily have to but often times in rock and pop music it can be helpful to play it anyway to fatten the chord a little.
I would like to add that slash chords can also be easy ways to accomplish some cool jazz voicings and is an entire topic taught by lots of jazz teachers. Check out Mark Levine's jazz theory books, there is a whole chapter dedicated to slash chords. One such example is C/D would spell out a Dsus chord in a jazzy way. To start with that concept the chord should be a 4 note voicing, the 3 from the C triad and the D bass note. It probably won't sound as awesome with just the bass player playing the D against a big C barre chord, maybe even sound wrong.
I play bass in a Rock group and often try to incorporate jazzy chords but my band mates don't speak the lingo so I use slash chords to relate. In one song I put an F under a C9, which would be a sort of Fmaj7 sus4 but they would never understand that.