In a lot of cases, I think that would help. But in my case, the car cranked and started, but when I let go of the key, it died. I would think that, during cranking, even a 5 Farad cap would not be enough. But I COULD be wrong.

Certainly, in the case of less-severe troubles, the cap may very well work. For $50, perhaps this would be something for some people to try when having all these problems.
If I owned a shop that dealt with such issues, the cap may be a good trouble shooting tool.