If you still do regular water changes you are actually doing the same thing with taking old and used out and replace this with clean water.
The plants and the substrate also function sort of like a natural filter, biologically by numbers and volume of organisms it seems to be a better filter than a canister. Then we could make the statement in a planted tank the canister is mainly taking free-floating particles from the water. If you don't take this out by means of filtering this will finally sink into the substrate or gather on the bare bottom as brown slimy sludge. Similar to what you can see on the bottom in sumps filters. Even with a filter there will always form sludge on the bottom, without one a little more and a little faster.
Useful or not?
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It all comes down to husbandry actually, without a filter you might need to do a tad more.
If it all is about budget?
Have you ever reviewed the Hamburger Matten Filter method?
And there are quite some alternatives to think of to make it cheap but still very effective.
This video actually is a Biohome advertisement, don't fall for it, but the filter in itself is ok.
You can use a slab of sponge and make it cylindrical around the pump.
You can also plant the sponge up with mosses and a variety of other plants.
All HMF ideas.
