I don't. TLDR; the buffs are deceptively bad, and his individual damage is paltry compared to true Fighters.
The partywide +.1 to physical damage multiplier (not an actual multiplicative x10% as the spell description would have you believe) is paltry on Hard+, and bringing along another Warrior to wail on a mob for hundreds of damage in the Battlesmith's place (remember, Battlesmiths suffer a massive opportunity cost in terms of individual damage, due to receiving a far smaller damage per level scale) simply makes more sense. The armor buff is a decent complement to the Wizard's armor buff early game, but after that it's negligible. In exchange they get access to some fire DDs that they will in practice never make use of, due to all their mana going to buff casts. If you focus on a Battlesmith's Knowledge/Willpower to scale his spellcasting, he'll be even worse as a fighter and need to be in the rear/flanks - in which case, you might as well just have brought another Wizard.
My takeaway from the hybrids, and especially the racial unique classes, is: very very cool idea and the execution goes in the right direction, but falls far short of the mark in terms of usefulness/power/balance. Racial classes pay heavily for their perks, and the perks themselves just aren't worth what's lost.
My sole possible exception is the Sage. Sage gets access to all Wizard nukes, and a second nuker is very nice in many situations. Along with this they get Mass Power Heal and some buffs. But it's a tough road to get there, and I don't believe they get Power Heal, so you're banking on a very late game before they really come into their own.
My first win had a Battlesmith, and he just never pulled his weight. He was a subpar fighter who struggled to get the mana necessary to cast his buffs.
