The quick toggle is potentially misleading. In most fights where Ballad is a thing, I'm using Rook. Rook can't MP regen, so swapping is two-step: GCD, place bishop, GCD, promotion. Swapping back is just placing Rook, though, since Promotion falls off on its own. Then again, "most fights where Ballad is a thing" right now is Savage, and in A1S I just place and promote during jumps and A2S is Bishop's time to shine.
Maybe this a PC thing and/or Summoner practice but replacing the turret is a non-issue? Cast and toggle is at worst a barely clipped GCD and an entire GCD tick ahead of Bard song. Maybe it's significantly worse for controller? I'm also primarily thinking of freebie moments like during a jump where you get a full extra tick out of it due to getting to frontload the effect.
The downside is that BV doesn't affect the bard who sings it. That's not always a downside. ... In basically every other scenario that exists Bard has it better than Machinist when it comes to spot MP regen.
Okay, first off, how is that not a downside? Like, there's no upside to not gaining the effect of your own BV. It's literally 100% downside.
As for the better at spot regen bit ...how? The only way their regen is better if you're BVing it for 6+ ticks. Otherwise they're the same MP gained except Machinist regen was there a tick earlier. Again, the one angle already stated as in Bard's domain for this is 18s+ of boosted regen since their duration is strictly better. At the very base level of their regens, they're equivalent however. The primary difference is one is more button presses to enable (switch, toggle) but it's still going to be faster by an entire tick.
Besides that, I'd argue that "spot regen" and "using BV / Hypercharge on it" aren't the same thing but this is possibly a semantics point?
Even when Angary mentioned that you can drop a turret and Promote/hypercharge during a burn phase to get MP back, it's still, in my opinion, way worse than Bard throwing up a single GCD.5 to get ballad ticking for as long as you need it to. You're impacting your dps, sure, but you also aren't burning an cooldown in a defensive manner where you may not need the full duration of it. If you turn promotion off during single target burn phase you still now need to swap your turret and lose your Hypercharge. Bard just shuts off the song and you're back at full damage potential.
So how is it a DPS loss to burn Hypercharge Promote here but not a DPS loss to burn BV here? Even if you swap songs, you're most likely missing full buff period of damage so it was a loss anyway. I'm not entirely sure I'm following this logic train here of somehow one used on regen is defensive and the other isn't. Both cases sound like massive losses of maximum potential damage?
It also sounds like you're saying Hypercharge Promote vs non-BV Ballad and it's entirely confusing to me? Also worth note, if you do Hypercharge + Promote, you can let it attack once to get the vulnerability as well as the MP regen. Can even refresh it and have 100% uptime of both effects. Additionally, Hypercharge Bishop even single-target is better than base Rook so you'd just eat the 40 potency loss instead of switching early anyway.
If all you have against Bard utility is losing that GCD.5, you're really focusing on the wrong points. As a BRD/MCH you're there to be a battery when needed, which means your DPS takes a back seat during not-a-burn-phase.
Then this means you're missing the point. It's about getting it out there an entire server tick ahead of schedule. In any event of toggling turret vs casting song, the Machinist will be one entire refresh tick ahead. The lost GCD I was bringing up has literally nothing to do with your DPS. The talk about the speed of the activation is about finding the free time to generate a tick of either regen that on Bard would be awkward due to the cast time. I have no idea how anyone can say that a BV not spent on Foe's isn't losing out on a lot of damage while saying that Hypercharge'd Promote is.
I have nothing against Bard utility because end of the day, Foe's is better for me than literally every single possible use of Turret stuff simply because I'm a caster.
Edit: Basically, my take on the situation in general is that Bard is better at the long term big regen with BV but it's an awful use of BV. When it comes to short-term spot regen, Machinist is better due to them being able to activate it quicker for free pulses in fight lulls. If they're both just running until empty on non-buffed versions of regen, they're effectively the same thing.
