Different people have different goals.. I track my performance just as a way to gauge my improvement as a golfer. I haven't entered tournaments, and the only people I bet with are my buddies who know my game. So, to me, an official index is useless. I have a goal to get down to a mid single digit (5-6) index, so I keep track to measure my progress. I also like having the handicap differential for a round, factoring in course rating and slope, since I try many new courses it's a good way to judge true performance on a wide variety of courses.
In past years, I did track my index through the NCGA (Nothern California Golf Association). It's not like there is someone checking the validity of submissions at the course or for Internet submissions. On the other hand, most of the incorrect submissions I have seen were reverse sandbagging.. Guys not playing by the rules, taking lower scores than they deserved and getting their vanity handicap low.