Quoth bjoern:

Plus, if you couldn't change it , it would be terrible. What you put on it at level 4 would bog down what you'd like it to be at level 20.

In this case the ability to wipe it clean and make it something new is both RAW and RAI.
By the same argument, feats themselves are terrible, since the feat you take at level 3 might bog down what you'd want at level 18. Or, you know, you could just plan ahead a little bit and take things that will continue to be useful as you level.

Talya, why the assumption that scrolls will be easier to come by than spellbook access? If we can handwave away scrolls because Magic-Mart, then why can't Magic-Mart sell pages, too? And if we're not handwaving it away, then wouldn't anything which would cause spellbook access to be rare also cause scrolls to be rare?