The LSAT is an incredibly important filtration system. Is it a good predictor of law school success or eventual success as an attorney? I really have no idea. If you are smarter than average and you work hard, you can do very well in law school with a little bit of luck.
The attorneys who seem to find the most success financially are those rising stars at big firms, great entrepreneurship who build small firms and make money off their staff and associates, or brilliant salesmen. I know several PI attorneys who are filthy rich and they hardly do any of the actual legal work. I know one brilliant estate planning attorney who works at a medium firm and has a great reputation. He makes just over $80,000 a year (decent for the market). I know another estate planning attorney who is pretty dumb and procrastinates everything, but he manages to bring in clients who contest wills and he gets paid a 33% contingency on what he gets for them. His take home last year was $400,000 +.
I worked for a medical malpractice attorney while I was studying for the bar. He worked about 20 hours a week, but he settled a 5million+ med mal case. (Translation, he took home 1.5 million.) He shares a secretary with some other solos and pays $800 a month in rent.
My point is that financial success may have a lot more to do with your personality, work ethic, and even luck than it does with intelligence. So it's possible that the LSAT is a decent indicator if intelligence even if it's not correlated with law school success or career success.