Beelstarmon is meant to be a hard-play Digimon: her play cost is reduced by 1 for each [Three Musketeer] Digimon or 7-cost option card in your trash. Subsequently, she has an [On Play] effect that returns a 7-cost option card to your hand, and then plays a 7-cost option card from your hand without paying its cost.
Therefore at a worst-case scenario when you do not have any [Three Musketeer] or 7-cost option cards in your trash, she becomes a 5-cost to play LV6 Digimon (assuming we paid in full the 7 cost option card we play in tandem).
While a 5-cost to play LV6 Digimon doesnt sound bad at all, pushing so much memory to your opponent is! So we wouldn't want to do that unless it is a game plan, such as running Megazoo or Security Control decks.
Personally, I feel that because she benefits so much from having a trash full of either big [Three Musketeer] Digimons or expensive 7-cost option cards, it does not make a lot of sense to run her in a standard digivolution-based deck, which can easily choke when filled with so many big option cards.
Unless though, she might work in an option-centric promo [P-027] Metalgarurumon deck but that's topic for another day.