Steve Jobs said that A players hire A players, B players hire C players. I have a suspicion that something not dissimilar happens with LinkedIn endorsements. Who would you endorse as having skills you feel you are strong at? Personally, I'd only endorse people who are really good at that thing - the endorsements are a reflection on my judgement, after all. So, generally, I end up only endorsing those better at the thing than I am. To endorse those weaker than yourself is either to say that you're incredibly brilliant, or have low standards.
However, such endorsements aren't really helpful, either. If you're looking for a top X, why would you trust the judgement of a relatively weak X in comparison? I guess endorsement allows the creation of self-regarding clusters of Xers who all endorse each other, but I'm not sure that's helpful either.
It's a bit of a difficult system to avoid, too. You see messages like 'Do you want to endorse Simon Peyton Jones's Haskell skills?', 'Do you want to endorse the Pope's Catholicism skills?' and 'Do you want to endorse this bear's arboreal defecation skills?', and... well, it's hard not to (NB: I've only actually seen one of those). So, top people get stupidly obvious recommendations from people that don't really matter.
About the only useful case I can see is something like a 'recommend a plumber' - you see a few recommendations like 'I'm an Xer, I needed a Yer as I know nothing about Y. Fred did all the Ying I needed, and now I'm happy.' Useful for consultants to sell their customer-facing skills, but not much else.