You're not missing anything. Most people have no clue about the workings of haste/march and the different haste caps. I constantly see people asking for haste when they have double sv march + embrava up >_>
To be fair though, it starts to get complicated and annoying timing buffs around when things are up and down (you can't keep marcato and sv songs up all the time) so it's easier to just keep things up. If you're limited on the number of songs you have, you can always restrict yourself to one march during sv and make sure hastes are up.
Edit: Thinking about it seem to remember mathing it out before and coming to the conclusion you can't cap Magic Haste with Marches and Marcato, only with Soul Voice. I also kinda misread what you were saying at first, but I'm gonna leave this up anyways as there's an obscene amount of brds around that think for some reason using marcato caps your magic haste with marches. It doesn't. Save it for Minuet if you have 3/4 song, or ballad 3 if you don't. (Running with 2 4 song brds, I personally save my Marcato for ballad 3 on my whms). Or use it if you really don't trust your mages to haste, but brd is a pretty lazy job and that's something you can help out with yourself.
Unless my understanding of the way songs are increased during Marcato and SV, these should be accurate.
Victory March gives a base of 96/1024. Advancing March gives 64/1024. Each tier of march gives 16/1024. Magic haste cap is 448/1024. For the purpose of this post, March +5 is assuming Horn+AF3, March+4 is Langeliek+Af3, March+3 is Piccolo+AF3, etc, you get the idea.
Normal marches(this is counting Victory + Advancing without any marcato buffs and sv):
March+5X2 = 320/1024
March+4X2 = 288/1024
March+3X2 = 256/1024
How much haste a Marcato'd Victory March gives per tier:
March+5 = 176 + 88(marcato bonus) = 264/1024
March+4 = 160 + 80(marcato bonus) = 240/1024
March+3 = 144 + 72(marcato bonus) = 216/1024
How much haste a Macarto'd V-March and Advancing march would give:
March+5 = 264 + 144 = 408/1024
March+4 = 240 + 128 = 368/1024
March+3 = 216 + 112 = 328/1024
I'm sick and my brain is a little addled, so feel free to correct my math.
Soul Voice'd double marches:
March+5X2 = 640/1024
March+4X2 = 576/1024
March+3X2 = 512/1024
Soul Voice'd Victory March(all these can cap [or near cap in the case of +3] magic haste with either Haste Spell or Embrava up):
March+5 = 352/1024
March+4X2 = 320/1024
March+3X2 = 288/1024
Obviously, any of those numbers lower than 448 need either a Haste or Embrava to cap magic haste.