I'm not sure whether a filter is usually a good idea or not - most flying is done in clean air, but on the ground and during an occasional dust storm the engine could ingest enough dust to wear it out quickly.  I would use a filter unless I expected to overhaul the engine before TBO for some other reason anyway.  However, lots of people run filters that quite a bit below the filtering efficiency of a standard paper element (oiled foam or K&N) and I don't know of any widespread problem.  Maybe the standard paper filters are more like insurance policies - likely never needed, but once it a while they are worth it.  I think there is enough dust right at ground level that I think a good paper filter is worthwhile.  Just an opinion, as I have no data to support it.  I can detect a trace of dust in the filter housing ahead of the filter, but after the filter it is clean.  The engine is expensive enough that I figure the filter is worth it.

Filters do inhibit solid water - sometimes too much - and like posted on this list the engine has no real problem ingesting water.  They also (presumably) prevent rocks, bolts and birds from entering.  A screen could do that, but I would hesitate to put a screen flat across the intake - something like a rag could plug the whole thing.  If the screen were cylindrical shaped or otherwise not in direct line with the flow it would offer enough area to hopefully not plug if something entered.

Gary Casey