I ignore them ...
I don't click the 'add to profile,' and thanks for letting me know this is a reciprocal thing designed to boost somebody else's 'numbers.'
LinkedIn *used* to have value, but lacks it now. In addition to the current endorsement lunacy, it appears there is some sort of unwritten 'challenge' to see how many people you can link with. It's another copy of 'how many "friends" can you have on Facebook that you don't know' (and no, I'm not on Facebook, either).
I've surmised that many I contact for work-related issues apparently have their e-mail set to automatically create contacts for those not already included in their addresses. Yes, I get both LinkedIn and Facebook 'invites' from them .. over and over. Do I detect a bit of trolling there? I've often wondered if this is with, or without, the knowledge/consent of the individual.
Recently, I sent a rather scathing message to LinkedIn regarding the downhill trends and growing lack of value to me. Have I received a reply yet? Of course not.