Most of the fast attacks are impossible for humans to dodge by reaction, due to our reaction time having a slight delay. If you don't use shield, you'll be heavily limited when it comes to defensive options. You can predict an attack and spot-dodge, although this won't work against lingering hitboxes. You could roll, although rolling too much leaves you open to be punished. You could short-hop air-dodge, although this approach is somewhat limited as well.
Not using grabs means your punish game will be quite limited. You'll be much worse at punishing the opponent's shield, and you might be forced to use quick aerials or jabs and tilts for punishes, which generally adds up to much less damage compared to grabs, especially if your characters has a combo throw of some sort.
Of course, you are free to not ever grab or block if you wish to do so. If you're not playing in tournaments, and if you still do decently enough against your friends, then keep playing like that if you wish. Just don't be that guy who complains when others grab and block.
