I personally think having a couple shadow preists to the raid would be a great plus.  I have been playing with shadow/holy for a little while now and been switching back and forth.  I think that we havn't had shadow preists for so long that the raid hasn't figured out how to properly use them yet, for example having preists in proper groups to allow passive heals.  A problem i found though is that we don't have as many rouges and such to benifit from shadow heals.  It might actually be better to have other preists in the same group as a shadow to keep them healed.  I still may switch back, probably will once the expansion comes out to see if the new talents make since.