I think I started pre-soaking E6 cos on a couple of sheets on a very blank even blue sky I got slight traces of splashing, which I decided was possibly due to developer splashing onto the film when being poured in. Should not be significant but with the dev time only being 6mins it was possible. It was not recommended in the kit instructions (at the time, Fuji Hunt) but I tried it and never had the issue again, so kept doing it. I note that on my current Belini E6 kits it does explicitly recommend a pre-wash at the same temp as the rest of the development sequence.
I am generally happy to not bother with a pre-wash if using something completely mainstream - ie if developing say HP5, FP4 or Tri-X in ID11 or D76 etc etc. But for anything a bit more flaky, temperamental, or prone to issues, like Fomapan or Pyrocat or anything off-piste like that I will definitely be pre-soaking every time. I have never experienced any issues with pre-soaking, it costs nothing, potentially protects against unevenness for shorter dev times, and any theoretical reduction in contrast/development must be marginal as I've never noticed it.
I suspect that Ilford don't recommend it simply because it's probably not necessary for their largely bullet-proof materials, and it adds in another variable which isn't really under their control that the photographer could cock things up, and it adds another step, so they don't recommend it. For instance if you're going to do something daft like throwing in a load of tapwater straight out of the tap in winter, which could be only a few degrees C, then have that temperature difference cause problems when you put in developer at room temp, stuff like that. They are writing their guidance knowing that beginners are going to read it, and also knowing that once down the film rabbit hole experienced users are probably going to experiment and ignore the literature anyway.
