I know several people who have adopted kittens or cats that have infected their other, previously healthy, cats with FIP, which was one reason why I had our cat tested when he was adopted. As it was, he continually attacked our older cat, and the two had to be kept separated for three years, until the older cat died. FIP is a major problem in our area (over 20 deaths last year in one small town). Although the FIP vaccine is far from 100% effective (60% is the lowest I've heard), IMO it is better than nothing. The FIP vaccine (nasal drops) is only administered to cats with 0 titers (at least here), so the test is necessary if you want the inoculation. The vaccination is highly controversial, but people I know who've lost nonvaccinated cats now say that they will vaccinate all future cats when possible. Our cat only goes outside on a leash, but there are far too many free-roaming cats here (many of whom are neither vaccinated nor fixed) to take a chance on exposure.