yeah, i had an absolute nightmare with this about a year ago. i just assumed that the defualt soft weight copy and weight paste would do the job, but after spending about 4 hours tweeking the weights on an arm until it deformed perfectly, i found that it didn't work how i needed at all!!!
the only way i was able to get around it was by using a phoenix tools plugin that did what soft was meant to, but properly.
it does appear to be quite an oversite and i wondered how others got around this problem without the use of phoenix tools.
anyway, sorry i couldn't help, i know how depressing it feels when you relise you have to do exactly the same tedious job over again.
leach