I don't think anyone would advocate that you just pour some hot water in your mash tun, stir in your grain, then call it a day without ever measuring a temperature. You need to know and control your strike temps, but everything else you can dial in based on your own system. Mash temp is important, but there are many other factors that also decide body and attenuation in a finished beer.
And frankly, if you can't taste the difference between 2 beers, then there's no difference as far as I'm concerned, regardless of what the numbers say.
In the end, each brewer needs to understand his or her own system. If you want more body, then boost your mash temps by a few degrees, hold it for a shorter time, use a less attenuative yeast, etc. If your mash temp drops a few degrees during the mash, then maybe you need to mash your stout a few degrees higher, or bump up the flaked barley, or whatever. Easy enough.
FWIW, I mash barleywines at 147 for 2 hours, but they still end up with a body like a sumo wrestler.