You'll get a range of responses on this one. Pellet machines move a lot of air with their blower fans. So unless you're boiling off a LOT of moisture from a water pan (adding about a pint per hour) you're really not adding that much humidity to the air that the meat sees.
My
Masterbuilt vertical came with a small pull-out water tray (hard to NOT spill when pulling out) but it was mounted under the highest side of the slanted grease drip tray, far from the fire crucible, so was very inefficient..maybe a teaspoon of water consumption per hour. I'd suggest a 6x6 or 8x8 cake pan directly on top of the heat deflector--the largest that will fit--and pulled out to the front so you can add to it easily with just a quick door opening/closing. (Note the condensation thus formed on the underside of the grease tray may then roll down into your grease bucket. If that's excessive, you may have to put a strip of metal adhesive tape under the drip tray to catch the
water drips and deflect them back into the water pan on the right/lower side of the drip tray.
I'm finding a big flavor/moisture improvement, esp with chicken, with generous humidity and it helps power through the "stall" with large cuts of pork as well. I don't claim to have an exhaustive amount of data on this but my anecdotal evidence is making me dig a water tray.
I should mention I also have variable 50-300 ohm resistance in series with my fan so I spend most of my cooks blowing quite a bit less air than
Masterbuilt intended...another mod I like but is apparently rare with pellet grillers.
But stay tuned, you'll hear other opinions.