Given Raul's fondness for MMs and MI cartridges, I've never understood his outspoken aversion to MC step-up transformers. True, a stepup transformer will increase the magnetic distortions of an MC cartridge, but the larger magnetic circuit of an MM or MI cartridge has the same effect. I don't see why one approach should be favored over the other.
The reason you might want to avoid an SUT is simply that they are an added source of distortion with loss of bandwidth; ultimately an impediment to transparency. Also the loading of the output is critical for best performance (otherwise the transformer will not have flat frequency response and may also ring at high frequencies) and this will vary from cartridge to cartridge. Thus it is likely that most SUT setups are not optimized!
The flip side of the coin of course is if there is no SUT you have to have enough gain to deal with low output cartridges! In my system, even though we had the SUTs properly loaded, they were never as transparent as simply going direct. I found that if the SUT was in place I was simply less likely to play the system.
I am not saying SUTs can't be excellent- they can. I am saying that its possible to do better.