Engagement is not unlike an org's position on compensation. Do we want to be leading, lagging or in the middle of our sector? The ROI will be different for each org.
That said, one risk of not going after engagement aggressively is that that the org may fall too far behind on the engagement curve within its sector and community. This would likely have a negative effect on employer brand.
Orgs that sidestep engagement as trendy or only pay lip service will not be credible with employees past, present, and future. (A token effort will only get modest if any results, and reinforce those initial observations.)
Another factor: Being perceived as a relevant player on the innovation curve. Even when trends come and go, top talent (as well as middle talent of good character and performance) may prefer orgs that are 'in the game' and experimenting with the latest thinking. Engagement will look different, require different investments, and have different goals at each org. But not playing at all is a clear sign that either leadership doesn't get it, or doesn't care.