I think the problem is that LinkedIn makes it too easy and the incentives are out of whack. LinkedIn figures out what you may be good at, and then presents it as a button to all your contacts. Brain dead simple. That's fine, but the problem is that people are incentivized to endorse you, either because they may think it's something nice they can do for you (and doesn't hurt them and takes no effort), or because they may be looking for reciprocity. It's just a terrible system.