That'd make sense, but I still don't know why you wouldnt prefer Wrath + flickering + demon boots + SoJ as it compensate the fcr, the all res, and the +skills while being cheaper?
Well wrath is a fine prebuff item but I just shot some footage from P8 TZ96, I'm in the game for a half hour and only summoned 2x tainted in the whole time, prebuffing at the start. You're forfeiting +5 to blood boil to get that fcr/resists on shield slot, but then trading off 40 resists on boots to get +1 boil
So it makes sense to either play fast games where you don't prebuff, and use baalos as just a +2 tainted / +6 boil- or to play long games where you prebuff, and use wrath as a +4 tainted prebuff (or better yet, find a +5-6 tome), and keep baalos as +6 boil
In terms of fire resist, the real tradeoff you make without full godly charms/ammy/rings/boots for max resists is to sub flickering flame on your merc instead of cure. You're really just trading cleansing aura for +38 on resist fire (107 instead of 69 on tainted)
That's why on bnet I'm just bothering to use more 11 fire resist scs (not even lifer, just some plain ones), use a cure setup and still have 125 fcr, max fire/light resist and max damage.
Now I will say, there is a LOT of areas where you don't spawn immunes and its legitimately not worth casting blood boil at all, the fastest way to clear it is to just teleport around quaffing mana potions, and heck you could self-wield insight in those areas (and I'm killing P8 games in OHKO with 2x tainted, don't need the 3rd, so nothing lost by keeping up venom lord). Then it doesn't even matter what tome you have. But in the fire immune areas it definitely matters to have max blood boil damage, particularly the physical portion that comes from +skills only and mang song + baalos is 11 skills, obsession + wrath is 5 skills