why go against REL setup recommendation?

A REL ST series sub is not a conventional subwoofer; it is a "sub bass system". Many that talk against corner placement are speaking in general. I believe the REL/Sumiko instructions start out by saying to forget all you've ever learned about subwoofers before setting up the REL. So they are probably aware of the "conventional wisdom" out there. There will always be a room where the corner does not work, however the instructions even have a couple possible solutions for boomy corners.

A REL ST series subwoofer is not designed for playing mid-bass along with, or in place of the main speakers. It is meant to play below the main speakers playing full range (a true sub bass unit). If you want to use a REL like a "conventional" subwoofer, then buy one of their Q Series models.

I read some Audiogon posts where the person goes against all the REL's and Sumiko's recommendations; and wants to use the sub against its design; and then complains the sub does not work well. We American's don't like to read instructions; we are too much in a hurry.

Whatever.....It is not my assertion; it is REL and Sumiko's. I did say there will alway be a room it does not work (and so do they). It is more a case of folks (not Sean) who never even try the corner, because they have already been brainwashed to believe otherwise.

Best to start in the corner, since REL recommends it, even if it does not work in the end. If it does work as intended, the corner is also usually good from a standpoint that the sub is out of the way of the system and other things in your room. Put a plant on it :-)

Generally speaking, I find it interesting when people say the subwoofer "sounds better" here or there. A properly integrated subwoofer for 2 channel audio is sonically invisible. It should not call attention to itself.

The specific recommendation is in the corner behind one of the main speakers. I suppose if your speakers are out in the room, or part way along the longer wall, then the sub placement may change also.

A lot of REL's market is outside the US, and mostly only us American have these large dedicated media / listening rooms. The III series REL's are very adjustable. You should be able to adjust the sub to the chosen placement in most cases, instead of adjusting the sub by moving the placement.

Many folks have only one place for the sub in a multi-purpose room, or those with WAF placement issues.

Once I get a subwoofer properly integrated for 2 channel audio it will actually sound good placed almost anywhere in the room, simply because I can't listen to the just the subwoofer. It is blended perfectly into the music. It does not call attention to itself one single bit.

I can only "hear it" when I forget to turn it on, and start to wonder why the music sounds a little flat today.

This was Richard E. Lord's (aka REL) goal in designing the ST series. If you like to listen to bass, I agree, save your money and get a cheaper REL Q Series sub, which is a more typical subwoofer design, but still very musical in its own right. Or choose another brand.