Maybe your particular development environment(s) don't use function keys, but I find function keys indispensable when debugging. Most debuggers I use have the function keys mapped for step-over, step-out, and step-into.

The editors and IDEs I use also generally use the function keys for code-search, goto-defintion, and symbol-rename.

For me (and I suspect many others), the lack of tactile function keys is a productivity hit.