Whenever I think of cracked pottery I am always reminded of the anti-bullying lesson that you see from time to time on the internet. A teacher tells her students to take a sheet of paper and crumple it up. Next she tells them to uncrumple the paper and smooth it out as best they can. She then points out that no amount of smoothing is going to get rid of the wrinkles in the paper and that this is just like what bullying does to someone. No matter how hard you try, the damage is there and always will be there. I think that no amount of filling in and covering up is ever going to get rid of the fact that there is a flaw in the vase or bowl that you are trying to fix. It's much better to address the root cause of the crack be it trying again with a new clay body, drying things out more slowly, or just using better construction techniques. There's an important lesson to be learned in that crack and working to correct the root cause is going to teach you a lot more than just filling in a crack.
Now, this isn't to say that flaws and cracks can't be a good thing. Look at Peter Voulkos' work. His pieces are intentionally filled with flaws and are all the more beautiful for them.