The thing to remember is the Skelton brothers aren't normal. They are freakishly large human beings. They aren't 140-150kg's just because they are carrying a bit of fat (though they could do with more conditioning work), they have absolutely enormous frames. Will Skelton makes someone like Patrick Tuipolotu (who is pretty big in his own right!) look tiny in comparison, and this not just because he is carrying a bit more fat...
Just as a nice visual example of how freakish the Skelton brothers are here is 13 year old Logan (on the right):
You only realise just how big he is when you discover that the "small guy" standing beside him is Waratahs number 8 Tala Grey, who stands 1.94m and is 114kgs......
Players optimal playing weights depend a lot on the individual players. Some players are naturally slim, some are naturally a lot bigger. For example if we think of NZ locks over the last decade or two we have seen quiet a variety of different body shapes. The likes of Norm Maxwell and Ian Jones were tall and thin and had an ideal playing weight of around 100-105kg. No matter how much you tried they were never going to be near as effective at 115kgs. In contrast the likes of Brodie Retallick, Patrick Tuipolotu (and a bit further back Royce Willis, Brad Mika, Mark Cooksley etc) all had much bigger frames, so had ideal playing weights much closer to 120kg. My point I guess is that with the Skelton's freakishly large frames their ideal playing weight will be much higher than a normal locks - 140-145kg may indeed end up being the ideal playing weight.
In terms of how effective they will be I do agree that they are never going to be high work-rate players, and I do think it can be a liability having a lock that can't be lifted in the lineout. However as ratsapprentice points out Will Skelton is an ideal impact player. Like Larksea personally I prefer the sort of lock that spends the match doing his 'core roles' - jumping in the lineout, making his tackles, hitting every breakdown, but I do see a room for an impact player like Will Skelton in the modern games, though teams do have to make adjustments to accommodate them...