I have always used sumps in my 30 years of keeping fish and corals. Not always using a trickle filter however. I am not planning on using one on my planted tank and wondered what every one else is doing.
I answered no however I don't rely on the plants as the only filtration. I use a canister filter to take care of mechanical and biological filtration. The plants assist and most likely compete with the filter for biological filtration however I would not run a tank without a filter.
Sumps are fine for hiding equipment but they have the large drawback of causing too much gas exchange which is unwanted in a planted tank, specially if you are using CO2. I wouldn't run a SW tank without one but not a planted tank.
For 6 months I tried to make use of an expensive sump that a friend of mine had on her tank.
The sump had bioballs and a sponge to filter big particles.
We injected the CO2 in the intake of an Eheim 2126 canister filter (the one with the integrated heater). No bubbles came out of the spray bar of the Eheim - we assumed that the CO2 was being disolved 100%.
But the sump would let all the CO2 go. pH never dropped below 7.0. Even after removing the drip plate and sticking the hose coming from the tank under the water (among the bioballs) the loss of CO2 was still 100%.
The pH dropped to 6.6 in the matter of hours after removing the sump and hooking up a HOT magnum to the tank to keep the circulation to the previous levels.
The above is only my experience. I've heard of people succesfully using sumps with planted tanks.
I like my sump. With Co2 gassing off, you just need to play around with it. But I think you end up using more Co2. Make sure your flow rate is not to high and that you have a good reactor so that you get a good mix going into the tank, because it will be gassed off when it goes back down the outflow. I have my inflow high up on the tank pointing slightly down toward the front glass so that the current draws the Co2 down in a circular motion. I use it mainly for mechanical filtration, and stick filter floss where the usual sponge sits and micron bags around the pump (try to polish my water). Benefits of sump,
-higher O2 levels help with some algae
-constant water level
-hide all you equipment
-place to grow floating plants
I have a sump, and I rely on the plants for filtration. So I guess your Q's are invalid in my case. I love the sump as it's an excellent place to dose, hide equiptment, maintain more stable parameters in the tank via osmolator ect. I do use about 3x the CO2 I used to when I had no sump however.