In reply to graeme jackson:
They definitely don't have that effect on me. If that's what they do to you, you probably need a bit more fibre in your diet generally. You get used it it.
As to the rest of the 'why' - they're very tasty, last for ever without refrigeration and are ridiculously easy. Only downside is that they're not dried or anything, so relatively heavy if you wouldn't be carrying the water anyway.
Same benefits as the "Look What We Found" thingamabobs in short, but suitable for veggies.
Also, I don't generally find it particularly traumatic to take a dump whilst camping. Here's a book recommendation for you: www.amazon.co.uk/dp/0898156270