Rex,
I believe Brightspark Magnetos is very forthright regarding why an EasyCap might be a good choice. They recognize other potential causes of failure while noting that capacitors are the most common cause. The text below is clipped from the Brightspark website. My EasyCap made a difference in starting when hot. I would not consider anyone pondering an EasyCap to be shortsighted, or consider Brightspark shortsighted for seeing the need. Let's say you are pondering a complete mag rebuild or just an EasyCap. In my opinion the complete rebuild should have an EasyCap. So, you buy an EasyCap and test it in the mag off the bike using a drill motor to spin it (the mag, not the bike). If there is suddenly very good spark where there hadn't been previously, Bob's your uncle. If not, then, go for the complete rebuild. Bob's still your uncle, just not your rich uncle.
Although many things can be the cause of failure, particularly when hot, capacitors are the commonest cause of 'internal' magneto failure. Despite their modest cost, replacing them in the traditional way requires time and a certain amount of skill. Often, it will only be undertaken in the context of a full overhaul and rewind of the coils.
Because capacitors fail more often than HT windings, our objective with the EasyCap has been to separate capacitor replacement and maintenance from the 'full works' rebuild/rewind, while at the same time empowering the magneto owner to tackle the work required for the initial fitment of one of our EasyCap products.
Richard L.