That's a different kind of drill. Using one doesn't exclude the opportunity for the other. Except for the occasional swipe to give you something to duck, focus mitt work doesn't include "borrowing" from the opponent, either.The issue is if your opponent doesn't resist (such as block your punch), you can't borrow his resistance force.
What is "combo" training? IMO, combo training is you make a move, your opponent responds to it, you then respond to his respond.
A good training partner needs to provide his opponent opportunity to train "respond to respond". For example.
- You right punch at your opponent.
- He uses right upward block to block your punch.
- You use left upward block to re-block his upward block (help his upward block to move even higher).
- You then change your right punch into an upward elbow strike to his chest.
In order to train this combo, your opponent's upward block is required. Without it, your right straight punch that change into an upward elbow strike won't make sense.
I use focus mitts sometimes, and see this drill as being similar. I also use drills where the uke (opponent) counters, and it flows to a next technique. That's not a better drill - it's a different drill.