There was a court case here in October brought by a woman called Debbie Purdy who wants the right to die in a manner of her own choosing. She has MS and is unlikely to be able to travel anywhere without assistance or indeed take the necessary drugs.
She lost the case she brought in the High Court where she sought a judgement that would prevent her husband from being prosecuted for assisting a suicide. This is, apparently, still an offence here. You can get up to 14 years for it.
See this link
http://news.bbc.co.uk/1/hi/health/7697200.stm
So I guess the answer to your question is that by doing what he did, he was able to prevent his family being prosecuted here. Although some mean-minded people might suggest that just by helping him travel there they render themselves liable to prosecution.
My father died last year from Motor Neuron Disease, and it's not a nice thing at all. If he'd suggested doing something like this, I'd have done anything to help him. Including spending three grand, if that's what it cost.
I figure you can't know what you'd do until you're in the situation.
Dignitas is a charity registered in Switzerland and, as such, won't make a profit.