Perhaps the strongest example I know of the advantage of cross-training on running would be of Dean Karnazes. If you don’t know about who he is, that’s totally okay, Google/Wiki knows a lot and would love to tell you too.
The whole purpose of cross-training is multi-fold. It pushes your non-running muscles which in turn makes you insusceptible to injuries and allows you to run faster and farther. People do all sort of things, from cycling to yoga to Pilates to strength-training. My typical Thursday includes a game of badminton (1-1.5 hours). So, I am perhaps a bit on the conservative side. Apart from that, I rode a lot (well, a little). ~8k in the morning and ~10k in the evening.
Update: I have a half-marathon coming this Sunday. Unarguably, I should take it as a weekend training long-run rather than a race-run if my racing instincts would let me.