myself i don't but that's because i rarely do spells anymore, or if i do candles aren't needed. though i don't think i've heard about only using a candle once. maybe if it's candle magic and it need to burn away, then i understand why you shouldn't re-use a candle, but you could try cleansing the candle and re-use it if you didn't need to let it burn down in a previous spell.
I tend not to re~use candles, candles hold the particular energy you are drawing and focusing into it for the specific spell you are doing at that given time.Most candle spells can be done over a number of days thus the same candle needs to be used, or in some cases the candle needs to be burnt down fully for that specific spell.
I get a little paranoid about mixing up energy, mixing up intentions, so I use a new candle for each different spell I do, unless the used candle is meant to be symbolic of something.
Its like in a hospital when doctors try to stop contamination..... I don't want to mix the spell I'm doing with a spell I did last week or something.