I didn’t realise I need this until I started trying to do pieces where one track keeps going at the same beat, while another does an accelerando or ritard.
I’ve seen other people post this and I thought it was only for arcane classical music, but I’m now convinced, it’s necessary for a lot of other genres.
When you’re workgint with live players it’s a non-issue, but with MIDI, you have to have multiple tempo tracks to do anything like what I’m talking about,
+1
It will be a nice feature and it would justify to have tempo tracks the way they are now (movable, instead of linked to the main ruler).
With several tempo tracks (and time signatures as well) you can do Medieval music and vanguard stuff.
So +1
+1 This could be handy for managing different sections in film music
Does MIDI 2.0 accommodate multiple tempo tracks? If so, you probably only need to be patient. If not (which is what I would expect) there’s probably no chance you’d ever see this feature until the MIDI specification defines how MIDI files are to retain such information. Not that this is an idea without merit … but I’d certainly hate to be the developer tasked with implementing it! 
IIRC, MIDI 1.1 supported a feature where each -track- could have its own tempo. I dunno if/how any sequencers utilised that. The files I recall had the tempo map as the 1st track in the MIDI file with multiple tracks.
But that was a looooong time ago.
—JC
I used to work with Opcode’s sadly missed Studio Vision which allowed multiple, independent tempos. In fact you could do all kinds of crazy with multi sequences all running simultaneously with completely independent tempos, time signatures etc…
I remember that. It’s an interesting sort of idea generator.