With out going forward it will make it hard for the horse to perform(dressage moves, jumps, ect.)
But you also need to be sure that you don't confuse speed with the impulsion. You want impulsion, and you want to be collected.
Yes, forward is one of the most important things you can teach a young horse. Without it, you won't go anywhere, either in your training or in terms of miles :p
Without forward you cannot have relaxation and rhythm.