I don't understand how people can be so dismissive of LinkedIn purely for its resume function.

For essentially every "knowledge worker" profession with a halfway decent CV, a well kept LinkedIn resume can easily make a difference of $X0,000 in yearly salary, and the initial setup takes one to a few hours. It's one of the best ROI actions many could do for their careers.

How dismissive many engineers are of doing that and the justifications for that are often full of privilege.

You have a special loathing for a site where you can message professional contacts when you need to?

Nobody is forcing you to use the social networking features. Just use it as a way to keep in touch with coworkers.