In a past life I was an academic researcher. My objections are nothing specific, I just never had any obvious uses for it and I'm deeply dissatisfied with the centralized Facebook model so to speak.

ResearchGate seems to blend Facebook with LinkedIn and arXiv. That's great for finding and applying to open positions, but not so much for casual discourse. There's an entire conversational continuum, ranging from things I want to openly share with a prospective employer to crazy ideas discussed with one or two coworkers.

For small semi-private conversations, we have email (and dozens of other services and protocols, each with various pros and cons). For large group communications, we have mailing lists and forums of various sorts. For LinkedIn, why wouldn't I just use LinkedIn itself? There's also ORCID, among other things.