In reply to BedRock:
Berghaus PacLite works fine for me. Mine is about 8 years old now and coming to the end of its useful life (the lining is starting to peel away in high-wear areas), but has generally been great in all conditions/ seasons.
90% of the time you will only need one jacket. But if you're going for two then I would say for MTB'ing you need something a little more robust than running. Running you can get away with a super light 'shower-proof' packs-to-the-size-of-a-golf-ball type thing, whereas MTB you tend to get a bit cooler (especially on the downhills), so something actually waterproof is best.