It's extremely common do do a full round of marches/minuets and then replace the minuets with minnes on tank in C.
Assuming your current plan is:
Gjalla Minuetx3, Daur 2 march, gjalla vmarch, marsyas hmarch
gjalla minnex3, balladx3
New plan with MC (which I wouldn't use because it's less potent but w/e):
MC Minuetx3, daur 2 march
MC minnex3, balladx3
MC vMarch, marsyas hmarch
All of your characters have the songs you want, and you have cast exactly 0 extra songs and used 0 extra piannismos.
We also do a full round of marches/minutes/dirge and rep dirge on tank with sirvente in v25.
This works without any changes using MC, and more reliably. Cast Dirge last, then piannismo Sirvente on tank and it will replace his Dirge. Unless you're implying that you have Hmarch on your tank for some reason, in which case cast hmarch last. If you give me specific songs I can also solve this one without adding extra song casts.
There's countlessly other scenarios where you want to just rep 1 song on 1 player.
Where that person also needs Hmarch and you can't cast it last?
Ofc in the scenario you listed it doesn't matter id you don't mind taking much longer to get songs on.
Could you point out anywhere that extra songs have been added to any setup? You have to use your brain to figure out the correct order to cast in, but it never requires extra songs, at all.
I guess if, for some reason, you insist that you must cast Hmarch before putting on your dummy songs and also insist that it must be on your tank and healer for some reason then sure, I concede that you might be adding *1* song to your entire buffing routine by using MC.
If you're not willing or able to plan out your songs before an event or think critically about the fastest way to do your buffs, sure it's not a good instrument, but with proper planning and gameplay, it offers major advantages.
And once again, I think that MC has very little place outside of SV use. I would be very hesitant to use it in the first half of a segment farm for example and I would NEVER use it in a Gaol fight...but you absolutely cannot deny that it has a place in a good BRD layout.
Let's say you're doing your plan of Minuetx3, March March replace Minuet with Minne. What if you get slept after the Marches and can't wake up until after Nitro has worn off? Now your tank has 3 Minuets you can't remove for 6 minutes unless you cast 3 minuets in dummy gear and then 3 minnes in your true instrument AND those still only last 5 minutes, completely *** up your duration. Well, with MC you can just plop 3 Minnes on them and they will take the place of your Minuets and last until your next Nitro.
The mental gymnastics required to say MC is not useful are...quite impressive to say the least.