Strange Filters were integrated poorly. I know of nobody who disagrees with this. They should have been like strange parts, adding a prefix and a subcounter showing how many kills you’ve gotten on a single map. Lobotomizing the actual function of the strange weapon on 99% of maps is not the way to get people to buy them.
This article by Brokkoli makes a good case for strange filters by pointing out that it makes your weapon truly unique. Indeed, you are unlikely to ever see a Strange Rigid Market Gardener for the rest of your TF2 career, assuming you made one of those. I’d like to add that this can work best on strange weapons which track things you don’t really care about. As much as I love the Pain Train, I don’t like how it is completely useless in certain game modes, or on defense of even more, meaning that there are many times I won’t bother equipping it. In addition, its upside has nothing to do with killing people, so I see very little reason to bother advertising how many kills I’ve obtained with the thing. Instead, by adding an amazingly-thematic Freight filter, I elevate my Pain Train to something truly memorable.
But there’s a great use for primary weapons, and it ties into how the strange filters were coded. When added to a weapon, they hijack the primary counter but not any subcounters added via strange parts. Add a strange filter to a Rocket Launcher with “Dominations” and you can still count your dominations on any map. And really, isn’t showing off your number of dominations far more impressive than your number of kills? Really think about your favorite weapons and see whether a strange part can highlight the skill in your kills.
The best use of this is on melee weapons. There’s a cheap-as-dirt strange part called “point-blank kills”, which you should realize is every melee kill. That’s how I got a Strange Hydraulic Specialized Killstreak Festive Ubersaw and am completely uninhibited by the strange filter.
So I encourage you think beyond the primary kill counter and explore some possible ways you can demonstrate the true uniqueness of your playstyle and your priorities. Mapmakers aren’t the only players who can benefit from strange filters!