This is an important verse and is taken out of the Bible. Regardless of what Revelation says, this verse should reach everyone who reads.
When I looked up Mark 11:26 in Niv, i found a reference to :
Matthew 6:15New International Version (NIV)
15 But if you do not forgive others their sins, your Father will not forgive your sins.
So really, it is not missing from the bible then?
I was confused at first about your post, lol. After re-reading it, I'd like to say that just because one part of the Bible states a verse, doesn't mean it should be taken away form another part of the Bible.
One more thing. Two of the versions I have do not include the reference at all.
Moreover, why take away from the impact? Even if a version includes the reference, why take the scripture out at all? It's common knowledge that not everyone pays attention to the references. Many Bibles have NO references at all and are not study Bibles.