Well if you only use it for a few MUs then it would be a secondary, and would require less work than your main because your main would have to learn more MUs.
Of course you will have to put in work in, but if you're already good at universal things like spacing, DI, reads, conditioning, punishment etc. then a char with a simpler metagame like D3 won't require as much work as say someone like ICs or Pikachu.
Generally, a secondary will be less developed than your main because you put more work into your main, that's why it's your main. Your secondary requires less work because you use it only to cover a few MUs, pretty much all of which are in its favour. I'd have to put less work into a Marth which I only use to cover Ness, Lucas and PT, than I would into a Marth to cover high and top tier.
If the sec that covers the all the unwinnable MUs the best is a technical char like ICs or Pika, then yeah they'd be the best sec, but my point is if there's a sec that covers all the unwinnable MUs, and is simpler to play, or has a more similar style to your main, then that sec is more efficient because you can spend more time on your main.
But I don't really understand why you'd have a sec to a low tier (unless you just enjoy playing that char). If you care that much about winning that you'll pick up a sec, you might as well just main a top tier, especially since your sec is likely to better than your main.