having multiple filters also comes in handy if you ever need to instantly cycle a new tank, hospital tank, friends tank, etc.

another thing is that some filters with high flow are best for mechanical filtration and removing particles from the water column, while others like sumps are best for biological filtration. I run 8 filters on my 190g (2x ac110s, fx5, 55 wet/dry, and 4 sponge filters) and I wouldn't have it any other way.