Are Dust Covers a MUST

I like dust cover primarily for protection from Dust, prying hands or accidents.
As far as sound with the dust cover on,in my experience, it definitely changes the sound towards warmer side. If you had edge before, it would likely go away. If you had warm sound, it would change to warmer. It is almost like you changed your phono cable

Which one is right? The one you like!

I like it with cover on.

i fabricated a sliding cover for my turntable. the cover slides on rails attached to my 5 or 600lb rack. the turntable sits on a HRS M3 isolation platform which sits on a spiked shelf. the cover is usually open while playing but gets closed when there are people (including my 4yr old daughter) around. there is no impact to sound open or closed. you can see pics in my system.

The dust cover creats a resonant cavity. Always try the TT with and without the cover to see which sounds better.

Cheerio

"I like it with cover on"

I should add, 'thus far'. As I am still experimenting. I just got cover (custom shape) made and delivered on Christmas Eve. BTW my cover rests on TT plinth/stand. May be the cover acts as acoustic receiver and adds/impacts to the TT's vibration control (system).

I'm sure Gingko products are top quality but charging $300 for a piece of acrylic is absurd. Got mine custom made for $75 including cutouts for the tonearm that protrudes beyond the plinth of my Scout slightly when resting. The shop owner told me there is absolutely no good reason why an acrylic dustcover should cost $300. No kidding.