1. When you melt a candle using a warmer, it melts from the bottom up, so you have to melt all the wax underneath before the fragrance gets released as the top wax melts. So, it will take much longer for you to get any fragrance out of it and since you are melting ALL the wax each time, you will get a lot less hours of fragrance out of the candle than if you had burned it.
2. When you melt a candle on a warmer, the wicks will usually sink down inside the wax or they can move from side to side. So, once you've used it on a warmer, the candle won't burn properly (if you decided to burn it later on) since the wicks won't be placed correctly.