I disgaree with Drake that the existing shards have set the precedent of only being useful in decks composed mostly of their primary element. SoRs have plenty of uses outside of decks, for example. SoSe obviously goes in rainbow decks of many kinds. A common / deck uses SoSac's with mark. Etc.
Which I think is cool. It's fine that Shards have natural obvious power in their home element but it would be boring/lame if they could ONLY be used effectively AT ALL in such decks.
That said I also agree with The_Tao's point that in general it seems like shards are taking over everything by being the most crucial components of the most powerful decks. You practically need sharded/unsharded as much as you need upped/unupped...
SoW, SoFre, SoD, SoR have the greatest links to their respective elements. After that, probably SoV/SoG/SoBe which have loyalty not to the element but to the mark. SoSe is (I assume) meant to be rainbow because of the nature of Entropy as 'the rainbow element' and SoI is only usable in a shardbow. SoSac is not very useful in death, but it's not very useful in anything but monolight and even then it's eh because of how splashable puri/sopa are. Speaking of which, SoPa/SoFo are far too splashable.
I am curious as to what the light/darkness SoSac and non-Time SoR decks you are referring to are. Just posted the above to give the rest of the thread some context as to splashability in the meta (for example, you won't see SoVs used as a main strategy without darkness mark, you won't see SoDs in a monolife, but you could see SoPa in a shrieker rush or SoFo in a monofire).