Is it worth building a personal brand on LinkedIn as an employee, or does it just increase the risk of competitors poaching me?

Yes, it's worth it, though the poaching risk is real and won't drop to zero. In our work with clients, the upside (stronger negotiating position, more inbound offers, easier pivots when you want them) outweighs the risk, as long as the company treats personal branding as a shared asset rather than a threat.