That's what I do. My primary backups are usually something like GK, DC/DR or DC/DL, MC/DMC, AML/AMC/AMR, and ST/AMX. Hopefully not more than 20% lower overall skill than the starter. My thinking is that I will only need that sub for:
1. the last 5-10 minutes of a game (to get the green pack bonus)
2. to sub in for an injury for the rest of that game only (it never takes more than 15 reds to heal a player no matter how bad the injury so I always heal injured starters 100% right away)
3. to fill in one game for a suspension
I play a 4-3-3 with 3 MC's and AML/ST/AMR, so for midfield I only ever need a MC sub (never need MR or ML). Also, I try to have one of my starting DC's also have either DR or DL and then have the sub with the other. That way any of the back four can be replaced with no one out of position (same thinking for up front). Flexible subs that may have lower ratings but won't be needed for more than one game.