I completely agree with Scott as to the two main reasons to not change the RRT:
Changing one core component such this in a system like ACES most certainly breaks its universality and above all interoperability benefits -- why bother using ACES (rather than any other "custom" workflow) then?
On a creative standpoint, one thing that I would like to state clearly is that, while the RRT may have an aesthetic look embedded in its scene- to display-referal conversion (and was indeed designed to have one), its function is purely technical, so the part where the basic "look" for a show or shooting is sought after can, would and should be placed in the Look Modification Transform (LMT ) component of an ACES pipeline.
To go into more detail for the first point above:
if you change the RRT you need to change all the Output Transforms as well, because all of them (or at least the "forward" ones) internally include the RRT as first "block". So, as Scott said, you will probably end up dealing with even more LUTs (and thus possibly more color-accuracy errors) than sticking with your long-tested (yet practically effective) workflow. So why bother using ACES at all?
To go into more detail for the second point above:
First of all please have a look at the official documentation about LMT: TB-2014-010: Design, Integration and Use of ACES Look Modification Transforms.
The LMT is a non-fixed component of ACES and, despite some LMTs are included into release versions of ACES (e.g. the "bleach bypass"), they are just meant as examples, while users can create custom LMTs as they prefer and even stack with each other. CDLs can be part LMTs, as primary grades do. They are all to be applied in ACES2065 colorspace (or ACEScc/ACEScct as working-space only), thus before any Output Transforms (RRT+ODT).
So if one wants to "unbuild" the RRT an LMT has to be graded or designed, that is stacked below/before the RRT, that hits the aesthetic goal after the LMT is viewed through the standard ACES output pipeline. It is guaranteed to be achievalbe without removing/replacing the RRT because of the invertibility of it.
But, ultimately, color-creative decisions should be left to DoPs and colorists, and color assistants and DITs, leaving terminologies like RRT and ODT just out of the equation since they are internal blocks meant to be referred to for technical purposes only.
Speaking about real-world experience instead, I must say that all the colorists whom I presented the latest versions of the RRT to work with since 2014 (but one) liked its default look and started to implement it from the beginning (using FilmLight Baselight ). The one colorist that didn't like it turned out to be just very acquainted to his own "base look", and actually asked me to change the RRT at the beginning (so I proposed to designing an LMT that also inverted the effects of the RRT and we started designing it together, both with formulæ n'graphs, and via colorist's tools).
The more he started using ACES in production though (grading in ACEScc and viewing in DCI P3 out of the projector) the more he liked its default look. So in the end it was him asking to stop this "deRRT" LMT in the end ― things I was verry happy to. I think it is very important that colorists I work with feel satisfied with all the technologies they use in the first place, including color management.
At the end of the story, however, we also ran a test showing to a couple of outstanding DoPs two different looks to start with and they all agreed to base-light the one with just the basic RRT applied ― and start grading from there.
As a last point, I want to say a couple of personal opinions as to the reasons why I believe that many facilities either used to work and might still be working with non-standard RRT.
First of all earliest pre-release versions of the RRT were highly non-invertible, which caused potential troubles to ACES' early adopters, i.e. important film post labs (like one I worked in for many years). Each one of these labs already had their own complete colour management solution, and had to stick to it due to color-alignment constrains with the photochemical process from film-scanning throughout film-printing. So early-adopting a pre-release version of ACES might have included custom RRTs.
Secondly, most of these labs (either they don't do photochemical any longer, or they do but I'm told ACES works much better with film now) are still sticking with their own color management workflow which has probably been successfully used for longer than ACES was released. This sometimes results in them exposing custom LUTs to external partners and contractors, wose management is yet quite cumbersome and likely feasible only if you also own a rock-solid color expertise, which smaller facilities usually don't have. That is where ACES comes in.
Said that, even bigger labs that were not early adopters are now all aligned to ACES, but sometimes smaller actors who are used to getting on with workflows shaped alike the bit labs' color management,, and based on cursom, unspecified LUTs, have some difficulty in changing the color approach.