Nothing wrong with not using a flag system. But you will catch more fish in some conditions by using a flag system. More so when running meat. If your running spoons or cranks you could skip the flag system.
If your flags are set right for the bait you are running. (I use rubberbands, not springs). You can see when a fish is biting/nipping the bait. the flag for an instant lets you know it is time to force feed the bait to the walleye for the many lite bite times out on the water.