If your tank is going to be planted with no livestock or heavily planted with a very low bioload from very little and small livestock then you don't need a filter at all...your plants will be you biofilter. That even includes any occasional dieing plant matter...the living and growing plants will suck up any ammonia that might be produced. The only thing you will need in the above scenerio is a powerhead or pump to circulate the water or a filter(to do the same) to provide mechanical filtration if you care or it.
Remember everyone has their own definition of "lightly stocked" - it's still best to recommend a biological backup. Heck I've had tanks with no plants done with zero - minimal filtration, but to recommend a filter less setup without knowing his tank size or any specs can be a rocky path.
You don't just consider the livestock, but I've seen planted tanks that are heavily planted to a point where the build up of detritus and leaf litter reaches ridiculous proportions.
Besides bio swings happen all the time, you want to leave some margin for error.
Sometimes the cheapest route isn't the best route, filtration is a good example - sure you can have a beautiful planted tank with just one pump, but what if you are someone who isn't always on top of his maintenance?
And I'm pretty sure everyone's term of heavily planted varies a great amount
, I've seen two swords and stem in a 20g considered heavy
. Also certain plants would be more demanding of biological filtration as they generate more waste, but again that's me speaking from personal experience.