where the employers have the majority of bargaining power.

Why? Most labor markets aren't monosponies.

If per-worker productivity can significantly increased by investing in optimization and automation, then yes, demand for labour becomes more flexible, and increases in labour costs would create more unemployment.

Or if the workers are doing something that adds some value but isn't strictly necessary, e.g., Wall-Mart greeters.

It seems to me that the gains in general efficiency of the economy would compensate for the extra costs of unemployment benefits to more people.

What gains in efficiency? After all automation and other capital investments cost money, and if it was a pure efficiency gain to invest in them, the company would already have done so.

You seem to be confusing being capital intensive with being efficient. These are frequently not the same thing, for example, a company that works with metal when faced with higher labor costs might decide to simply throw out its scrap rather than reprocessing it, this is more capital intensive (the company needs to buy more raw metal) but not more efficient.