I suppose in 98% of all cases you only want to filter what you have, but there are some cases, where you might need to set filters for items you haven't researched yet to fullfil future factory needs. And I think a filter is like a signal, if you can use it in the combinators, then why not on the filter inserters...
This is one of the scenarios: I like to pass red/green/grey/blue sience through the sience lab in one direction, pink/yellow/white in the other direction. Since I haven't researched them yet I can't configure my filter inserters properly from the begining:
PS: I know I can make a workaround with a constant combinator passing the filter-items as a signal to the inserters - feels a little bit unessessary though...
