Gardner's idea of multiple intelligences is something I agree with in part. There are definitely different aspects of intelligence. The human brain is just far too complex for there not to be. That being said, I really don't believe the idea of "all people are intelligent just in different ways."
I firmly believe everyone has something that they're good at, some area they excel in, but if you're a good athlete, according to Gardner that means you have bodily-kinesthetic intelligence... and I can't really get behind the idea at all.
The way I see it, some people are definitely cleverer than others and some people are just kind of stupid. The main thing, and the positive message in Gardner's theory that I really like, is that this is okay. You don't have to be intelligent, there's going to be something somewhere you're good at.
So what is intelligence? According to the literal definition "the ability to acquire and apply knowledge and skills". I'd add to that, that intelligence should also be extended to the ability to think logically and to use logic and acquired skills to solve problems/complete tasks with efficiency.
So if that's what it is, what good is it? Well that's the kicker I guess, the key word in that definition is ability. Like others have said it's talking about what you can potentially do. I think by that logic, intelligence is useful as an indicator of what someone might achieve, but is not as useful a yard stick as achievement. Using myself as an example, I like to think I'm a pretty smart person. I've got a high IQ, I think pretty logically etc. You could assume I'd be capable of good things. The reality of the matter though is that I'm yet to obtain my degree, I've only ever worked the one proper job really, I still live at home and I haven't contributed much to society yet. Sure, I might in the future, but am I really worth more than someone the same age who's less intelligent, works as a landscaper and just bought their first house? Fuck no.
It doesn't matter how smart you are if you can't get your life together. Intelligence is an indicator of potential, but it's meaningless in the grand scheme of things. Like others have said, merit is the truest indicator of success and societal importance. It's completely okay to value or be proud of intelligence, it's folly to assume that it equates to superiority.