Just saw this piece in Telegraph today, and found myself kinda nodding.
Much as I would LOVE this policy on all airlines (both financial & personal space/discomfort reasons below), I would though NOT be in favour of it unless it very specifically exempted people who had a medical condition confirmed by their Doctor that in ANY way contributed to their 'total travel weight' - whether that be permanant or temporary (IE all & everything genuinely medically driven from Thyroid issues to being disabled to simply being much heavier at that particular time due to say having just had no choice but to take steroids for a medical reason). As long as THAT exemption was in place, here's why I would welcome it:
1 - Biggest single variable cost of an individual flight is fuel. Every single kilo on board adds to that cost (hence my/you - pretty validly - having to pay extra for additional weight travel with). Why should my/your 'total travel weight' (IE me/DCs/luggage etc) be XY, but someone else's be XX, yet we have to pay the same? IE to put it bluntly, already be paying a frickin fortune (to point where maybe simply cannot now afford to fly) but part of that being to subsidise someone else's 'total travel weight' 
[NB do though think it is outrageous for any airline to try to charge for wheelchairs etc as that DOES seem like a genuine 'tax' on disability per se, so ANY medical items like that should NOT - IMHO - fall into any 'total travel weight' calcs.]
2 - Know I might get flamed for this but these are simple facts & not intended to offend anyone (& I can't see why they would TBH as THEY ARE simply truths/real experiences?). I used to fly a lot work-wise plus family hols etc. Lost count of the number of times where I was - literally - wedged into my seat due to the sheer physicality of someone else's body, and to the point of real discomfort.
Not even talking about the overspill onto the seat divider/occupying armrest for whole flight etc, but literally where someone else was spilling over their seat into mine - IE one of their legs squashed up against my thigh (as in, total body contact thigh to thigh with a complete stranger??!); one of their feet in the place where I was meant to be able to put my two feet; squashing me from shoulder down to armrest level where part of their body was simply on BOTH their seat AND on part of mine?
Thoughts?
< Only 2nd foray onto AIBU so donning hard hat
>