What's the purpose of combos, from a design standpoint? They seem to me like a way to show off what you've learned in practice mode, but it makes the game effectively one-player until wakeup. Plus it worsens the barrier to entry. Would a game without combos necessarily be worse as a result?
Combos do a few things! At the most basic level, they allow designers to connect moves together so that mastery of a character includes knowing how the character's moves are supposed to flow together. They also introduce meaningful choice if you have different combos to optimize for damage, stun, or positioning. They can give the way you play your character personality and flair. They can demonstrate your knowledge of the game's engine and your capacity to execute. And frankly, it's a parallel to actual martial arts that I think is kind of neat.
I've played games just for the training mode -- Bleach DS is a great example of a game with such a rich possibility space for combos that it's more fun messing around in training mode than actually playing other people.
However! They do come at the costs you mentioned -- higher barriers to entry, lower participation in the receiving players, etc. So I think intelligently designing a combo system means finding the sweet spot for your game between the costs and the benefits.
I've played games just for the training mode -- Bleach DS is a great example of a game with such a rich possibility space for combos that it's more fun messing around in training mode than actually playing other people.
However! They do come at the costs you mentioned -- higher barriers to entry, lower participation in the receiving players, etc. So I think intelligently designing a combo system means finding the sweet spot for your game between the costs and the benefits.