Unfortunately, most manufacturers of ergonomic keyboards seem to think along the same lines, so they omit the corresponding hardware keys and argue that you could always put the function keys on another layer if you really need them. I, however, use my function keys hundreds of times a day (mainly to switch workspaces) and I do not want to have press an extra key every single time.
The key principle I follow with all my key bindings is this: The more often I use a certain functionality, the more accessible it needs to be, i.e. the less work (or, literally, input) it should require in order to avoid putting needless strain on my fingers and wrists. (I have suffered from RSI in the past.) In particular, oft-used commands[1] should be available through a single key (or a very short and accessible[0] key chord) if possible. Unfortunately, the most common set of key bindings (Vim/Emacs/IntelliJ/…) usually violate this principle.
Long story short, this is the single reason why I haven't bought an ErgoDox/keyboardio/… so far.
Also, I don't think it really matters whether personally one uses a key X or not. In my experience, you can never have enough keys! And you will find an application eventually – provided the key is in the right place ergonomically speaking, i.e. easy to reach.
[0]: The more often I use a key chord, the stronger the fingers should be that push the individual keys – e.g. the thumb, index and middle finger.
[1]: Commands, in turn, that are used only rarely should actually not be available through a key binding at all – no one remembers them and they only clutter the key binding space and cause confusion when you do (accidentally) hit those keys. Instead, those commands should be accessible through a searchable command palette. (Think Shift-Shift in IntelliJ.)