Yes, setting a nuke off beneath something is a good way to get it off the ground, but the radioactive crap spreading all over and causing 7K extra cancer deaths per year presents a slight problem.
It wasn't stated as 7k extra deaths a year, it was:
Dyson did some calculations and found out that if Orion did two launches a year from 1960 to 1980 (or something) the result would be approximately 7000 extra deaths worldwide from radiation-related diseases.
So 7000 deaths over 40 launches, or 175 per launch. And in fact that's wrong too according to the Wiki page. It says:
Freeman Dyson, group leader on the project, estimated back in the '60s that with conventional nuclear weapons, each launch would cause fatal cancers in ten human beings from the fallout
So in fact it'd be 400 extra deaths over the 20 years. Worthwhile? Perhaps...