It's one thing to say that the way sumo wrestlers do shiko is "wrong" and useless for someone seeking proper martial training and conditioning. Professional sumo wrestlers train to use the whole body to move extremely large, heavy, and actively resisting loads, and they do a crap load of solo training meant to build up the connections in their body. But Dan is right about the sumo link above, in as much as it's a mere shadow of sumo training, meant as a commercial product for normal folks to get some exercise and to bring in some extra money to the Sumo Kyokai, with none of the heart of the real stuff meant for the dohyo.
Ultimately all these different takes on Shiko, "intent", ki, you-name-it, all *must* relate in a particular way to the cosmology stuff on A-Un, In-Yu, Ha-Heng, and so forth. These things were all developed and codified a long, long time ago around set principles and criteria. If someone in Sumo, for instance, is let's say "incomplete" in his Shiko, there is a set way for him to bring it back to "correct". If someone in a number of other arts do versions of "Shiko" (it's far more common than people think; it's not always called "Shiko" or done exactly the same way), whatever potential errors they have must be judged against that same ancient, core standard. In other words, just by "doing Shiko" someone is indicating that they are doing the ancient core principles and if they're wrong or incomplete for some reason, they can't say "in our style we do it this way". If it's not complete, it's not complete in relation to those ancient principles. So if someone says "here's the way to do it", but they themselves don't know the full old principles then they set themselves up to be easily exposed. Hence my suggestion that these things are probably best talked out before someone takes too firm a stance.
At the moment a lot of these things are just getting started and Shiko is a good component to think about and do. Personally, though, I think that people will ultimately understand these things far better if they understand the full set of principles, not just a few pieces. So if someone says "ignore the way so-and-so does his shiko", maybe it would be best if they put their reasoning down after having said something like that... it might start a good discussion.