I've used the hardened series extensively and had no problems except it being a memory hog as others point out.
At the end of the day, if the Tor and TAILS devs are stating it is not actually providing substantially more protection, then their expert advice needs to be heeded.
Dropping it as an option entirely in Whonix saves on maintenance effort for the relatively small team. If I recall correctly, the additional memory protections will also be incompatible with the Grsec kernel that should be vailable for Whonix templates in Qubes in the coming months.
So agree with your views and I should probably remove that recommendation in the Security Guide re: defaulting to the 'hardened' version for additional protection, since it is probably misleading.
Also, it is not clear to me whether it actually poses additional fingerprinting risks, since the vast majority of users probably do not use alpha versions of Tor Browser.