I thought Beast Within's main selling point is that it can pretty much always create a token. This can only create a token when there is already a creature in play, or there is a planeswalker in play. That seems like a pretty big limitation to me for an oath deck.
Using it on your own cards doesn't seem great either. I mean, I guess if a creature/PWer is going to be killed anyway, you can get a 2 mana 4/4 flier out of the deal. There are certainly better ways to get a value creature into play.
Using it against your opponent also seems underwhelming because leaving a leftover 4/4 flier is not a negligible threat. That can win in combat against many of Vintage's creatures, and is a significant threat to your life total / planeswalkers. There are certainly better removal.
I mean, if you are looking for a card that does all of these things, then sure it is an option, but I think you'd have be pretty tight on deck space to consider this. This is not much more than a niche Singleton.