Not worth it.
Beat Up deals damage from each Pokémon on your team that does not have a status ailment (nor is fainted). Each strike has a different base power, calculated as:
UsersBaseAttack ÷ 10 + 5
However, the actual attack is carried out by the original user of Beat Up, so it will always use that Pokémon's current attack stat to calculate damage. It will also receive any boosts from the originating user, such as STAB and any applicable items. However, any items held by the rest of the team do not affect the damage dealt.
Each strike of Beat Up is treated like a separate attack:
Each strike can get a critical hit independently.
Abilities that activate on contact (Static, Weak Armor, Iron Barbs etc.) activate for each strike.
If the Sturdy ability, Focus Sash, or Focus Band activates before the final hit, the subsequent hit will still deal damage and therefore cause the opponent to faint.
If the first hit breaks a Substitute, the second hit will deal damage to the target Pokémon.
The move Counter only counters the final hit, but Bide counters the complete damage.
Let's say we have the ultimate team of 1 Weavile and 5 Magikarps. One of the Magikarps is paralyzed, so only 4 can attack with Beat Up. Weavile's base Attack is 120, while Magikarp's is an incredible 10. Beat Up would attack 5 times, first with 17 base power (120 ÷ 10 + 5) and then 4 times with 6 base power (10 ÷ 10 + 5). The result would be as if Weavile had attacked with a Dark type move of 41 base power.
In a more realistic scenario, the average total base power for Beat Up is typically around 60-90.
- Okay-ish Base Power
- Other Party Pokemon generally don't get affected by Status all that much in-game.
- Good PP
- Breaks through Sturdy
- Much better substitutes for both Shiftry as well as Beat Up.
- On the off-chance that some of yourPokemon are fainted, it won't do as much damage.
- Abilities like Sturdy are rare-ish, and aren't all that hard to by-pass.
Tl;dr: Worth a try if you a want of fun, but to gimmicky and unreliable to be of as much benefit. Also, it is outclassed by a LOT of other options.
That said, you you try it once. That's where the fun lies, after all. Just don't expect to wreck teams left and right.