Flamestrike
Legend
It is confusing, but I think the RAI was to defeat, in part that 1 extra AC while dual wielding a shield and longsword. It sucks, I agree. In real life a shield is more versatile than a defensive tool. Shield master tries to approach that but keeps damage out of the equation. There had been a feat in the later playtests allowing shields to deal damage, but testing must have proved through that it was either not favorable or too many people thought it was overly powerful.
If a player was dead set on something like this, I would allow it. Who am I to stand between them and their fun?
And TBH, I reckon that was a snap judgement than based on any mechnical considerations.
For a TWF/ Shield build you need 2 feats (Dual weilder as the shield isnt light, and Tavern brawler for proficiency) and 1 Fighting style (TWF). Shield master, and protection style are also nice but not must haves. Thats a ridiculously heavy character investment. For most PCs that doesnt come online till 8th level (Fighters at 6th level).
Lets compare them to a different direction: 2 feats (GWM and PA master) and 1 Fighting style (Defence). Assumes Prof +2 and Str 16.
TWF/ TB/ DW + Sword and board:
- +3 AC (Shield and DW)
- 2 attacks (action and bonus action): Sword 1d8+3 and Shield 1d4+3, both at +5
- Can draw two weapons at once
PAM/ GWM/ Defence + Halberd
- +1 AC (defence)
- 2 attacks (action and bonus action) Halberd 1d10+3 and haft 1d4+3, both at +5
- Reaction attack for entering reach 1d10+3 at +5
- Cleave option;
- Power attack -5/+10 option.
- Reach
As you can see, for the same heavy character investment (2 feats + 1 F/S) the S+B option is no better than the Pole arm option. The S+B option has an AC 2 points higher, but deals less DPR, lacks reaction attacks, reach, cleave and power attack. It's defensively better, but offensively much worse.
There is no mechanical reason not to allow it in a game.
From a simulationist perspective it's fine as well (smashing foes with your shield is a thing), and it actually gives S+B fighters (the most historically common way warriors went into battle in any event) a leveller against the very popular GWM and PAM styles.
I think you'd be mad to disallow it personally. I can see zero reason for it, other than 'fighters cant have nice things'.