Could you please stop discussing SI Tendrils in this thread?
Generally I avoid playing cards against combo in the sideboard of this deck because you already have a good matchup against fast combo (I admit it is debatable how good it is but so far I have a winnig record against TES, IGGy Pop and Belcher). I'd much rather play cards that help in my bad matchups.
Of course there is an exception when the cards I use to improve bad matchups are also good against good matchups or are part of a transformational sideboard. For example Tormod's Crypt against Loam and IGGy Pop or the "man plan" that Bane of the Living is playing with Glowrider as additional combo hoser but most importantly as a creature to beat down.

The original Tabernacle is good as a one of but it has the drawback of taking a "spell slot" in the deck so it has to be tested what can be cut for it. Another factor why you won't see it in the majority of tournament decks is its availability. It took a friend of mine months to find one on ebay.