Joe Shmoe requested to add you as a connection on LinkedIn:
I'd like to add you to my professional network on LinkedIn.
and I have no idea who Joe Shmoe is. So I have three choices:
1. Accept the invitation and end up with a bunch of connections who I don't actually know. (Actually this might not be so bad. I have never actually made a useful contact through LinkedIn.)
2. Reject the invitation and risk offending Joe if it turns out I actually have met him and just forgot about it (which is likely -- I have a terrible memory for names).
3. Send Joe a message saying, "Um, who the fuck are you?" though maybe not in so many words.
It's just so freakin' annoying that LinkedIn would supply this completely useless default message that everyone uses instead of encouraging people to write a personalized message that goes something like, "Hi, this is Joe Shmoe. We met the other day at the underwater basket weaving society. I'd like to add you to my network."
For the love of Pete, people, a LinkedIn invitation is no different from any other cold call email unless you are absolutely positively certain that the person you want to link to knows who you are. You should put at least a little bit of thought into the content of the message before you send it.