is the trough a good idea ?
imo - the quick answer is it "depends" on how you have your ET2 setup, with what cartridge you are using.
Some history first
The first ET2's came out when MM's were popular. The same year CD's were introduced..... 8^0
Great timing - Still over 2500 out there. Imagine if digital was not around for another 5 years?
Anyway - So all early ET 2.0's had aluminum armwands. Bruce' tests with the damping trough reflect this armwand.
If you are using an aluminum wand, especially with later MC Carts, the damping trough is an improvement in resonances.
Years past I found the results very audible, and the amount of oil used directly effects how much damping is done.
The oil trough can be left on - you just turn the paddle screw in or out, so it is either touching or not touching the oil.
Note If someone has a Cat or Dog and or Pet, or other that sheds, the hairs, other floating material will find its way into the trough. The nature of the hobby. static, other, ensures this.
Later advanced ET 2.0 and 2.5 setups use different Armwands (Carbon Fiber and Magnesium), and different Leaf spring setups so they match up well with the Cartridge being used. This is discussed in detail on the ET2 tonearm owners thread. https://forum.audiogon.com/discussions/eminent-technology-et-2-tonearm-owners
This along with a properly isolated turntable, negates the need for a damping trough - imo.
My two cents. Hope this helps a bit. Cheers