Certainly an Elephant/Mammoth hybrid beat-down deck wouldn't be particularly competitive, but it would be fun. I wouldn't recommend investing in Elephant Graveyards for the purpose of building a casual deck around them, however, as they're $50-100 USD a piece for a decent condition copy. But if you happen to stumble across a copy or two of these for a reasonably cheap price, it's not a bad idea to pick them up.
Because Elephant Graveyard isn't really that viable to play anymore in any competitive sense, it isn't a problem for it to remain on the Reserve List. Keeping it there simply helps it retain collector's value, which for many Magic players, is perfectly fine.