Hum, I don't think the radius' usefulness has much to do with thermal. I think it is useful period.
In the day time, many hunters might be able to judge distance based on looking at the terrain. Around here, because most of our fields are on a grid system, called "sections" (which are 1 mile by 1 mile) our fields are typically divisions of the sections ... a sq 440x440 yards or 880x880yds etc. And when we reach a new field, we might use our range finders (hand held or otherwise) to construct a quick mental range card, like "That tree line is 660yds, the tree line over there is 330yds, that pond is 311 yds to the center of it, etc. And this sort of range card is fine for approximate distances, but might not be precise enough to shoot with.
So the radius can tell us how far away the critter is and if we've memorized our DOPE sufficiently, we can shoot quickly after accounting for wind and elevation.
With the thermal, perhaps what Hobbes is thinking of, it that under some thermal conditions, it is harder to see the terrain. In this "wash out" conditions (caused by wind, rain, ice, etc.) we might see "a sea of grey". Now the critters still stand out, but the terrain might be "suppressed" and hence, even if shooting on known ground you might not be able to see you usual "tree over there" ... or "fence post over there". However, most (but not all) thermals have an adjustment for this situation that helps a lot and that is variously called "gain" or "contrast". They are synonyms. My military thermals call it "gain" the civilians ones call it "contrast". But no matter. When I boot up a thermal, I'm set to middle of the range. By cranking up 1, 2 or 3 click I can add "contrast" ... and that makes a huge difference. Now the "sea of grey" is a "grainer" but non-sea-of-grey image. I can again see that tree and that fence post. I now have my bearings.
I hear many folks I talk to tell me, "when I got my thermal - I set it up for a good image and I don't touch it" ... well oh my .. maybe I used to do that, but I do not do that any more. I fiddle with my controls all the time. More than half the nights I'm out there (including last night where we had 20-25 MPH wind) I crank up the gain. Usually 2-3 clicks on the UTC or the Mk3 60mm. And without that, I would see sea-of-grey but with it I have my terrain back and by orientation.
I also adjust the brightness. Too much brightness can cause headaches (due to the mis-match of brightness in the two eyes) and reduce your natural night vision. So actually, you want to slowly dial down your brightness while you are out there. I start by clicking it down one notch in the first 5m while I'm nucing.
Oh and speaking of nucing. I nuc a lot. My guideline is 5 times in the first 5 mins. And then at least once every 5m thereafter. And I nuc when I can so I don't need to nuc when I can't (with target critter in sight).
And changing the pallet. When you first get a new thermal, especially one with a different core than you've used before. Experiment a lot with the pallets in different conditions and decided which ones work for you. I usually settle on 2-3 pallets that I use all the time and ignore the others, especially those FLIR "psycedelic" ones
I do "invert" which I see a critter or hot spot I cannot ID. And sometimes that helps. I am on black hot 98% of the time, but I switch to white hot, to try to help with PID on occasion.
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Now do we really even need range finders when hunting at night ???
Well, I am usually using 1 or 2 weapons these days:
5.56(10.3) IMI 77gr
7.62(22) FGMM 175gr
My holds for the 5.56(10.3) are flat out to 175yds ... then 6 inches up for 200yds, 12 inches up for 250yds and 20 inches up for 300yds ... after that I'm just peppering the target.
And for the 7.62(22) ... flat out to 225 .. then 6 inches up for 250 and 12 inches up for 300yds.
At 350yds 1.5 mils
At 400yds 2 Mils
at 450yds 2.5 mils
at 500yds 3.1 mils
Which is as far as I would shoot at a yote.
So, it matters for the 5.56(10.3) at over 175yds ... and it matters for the 7.62 and over 225yds ...
So far, I've shot 3 critters at over 200yds and a third at over 300yds. And that's out of 260 critters over the past 3 years. Almost all inside 100yds. So based on that, I don't need a range finder on my rifle !!!

That said, I'm trying to increase my distance and probability of kill on yotes and for that purpose, I think the range finder will help a lot. But I need a year of using it to prove it. The year has already started

call it 1 Nov 2018, the start of the year.
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So optimize your image every time you go out and continuously while you are out ... and know your DOPE and make your mental range cards for where you are operating ... and inside a certain distance you don't need to know the exact range ... otherwise, enjoy using your rifle mounted LRF to help with those longer distance shots ... I am !