The average Commander deck will prioritize Eternal Witness and ways to recycle its ability before they ever even consider Den Protetor as a back-up, but morph decks can use it as a synergy piece and likely have ways to reuse its morph ability to pick up extra cards
Except for determining whether or not a permanent can transform, a spell, ability, or rule that needs information about a double-faced permanent sees only the information given by the face that's currently up
For example, if the top card is a land, you can only play it on a main phase during your turn, and only if you havent already played a land that turn
In addition to ample Commander play, this card also sees use in both Modern and Legacy
That said, both of these rules were broken in later years, with sets like Commander Legends , War of the Spark (through planeswalkers), Dominaria , Dominaria United , Kamigawa block, and The Lord of the Rings: Tales of Middle-earth used Legendary cards as a mechanical theme, with uncommon Legends to boost up the numbers in Limited play
They're mostly harmless and appear very, very, very rarely, but they can provide you with some very rare and very powerful items if you poach them
