I use filters on all my planes. Two of my engines are pumped, and it is imperitive that junk not get into the pumping mechanisms and damage the diaphragms and other delicate parts. One one, the filter works as a place to disconnect the lines for fueling/defueling/running the engine dry.

Last week I gave one of my planes (my precious Venus II) a little overhaul and checked everything out. This included the fuel tank, which I removed and disassembled. Inside I found the stopper was deteriorating and there were little black bits inside the fuel tank. This tank was new last fall and had only been used for maybe 2 gallons of fuel. Filtering fuel from my fueling jug would not have stopped this from entering the engine, or any other junk that finds it's way into the system.

Filters like the Great Planes and Dubro filters have room for small bits of stuff to get trapped and still allow fuel to pass. They also seem to help with small amounts of bubbles that may end up in the system.

I recently started using the Sullivan Crap Traps. They have a dual sized filtration system, and are clear so you can see if anything is stuck inside without having to disassemble them.