A friend of mine came up with a theory on how the tomes would be necessary to have and how they could wear down quickly:
The text on the inside is not written in ink, but a powder that grants the user the ability to cast the spell. The powder slowly fades out of the book over time as spells are used, up until the runes are no longer existing on the page, therefore rendering the tome useless. This isn't to say that simple magic like conjuring a small breeze or lighting a candle can't be done without the tome, but the powdered runes in the book are what cause the user's powers to be amplified, depending on the type and concentration of the powder in the tome.
Does that make sense? Of course, this is just a suggestion, but it seemed the most logical to me.