Mostly, it cuts back on the resources required to gather information on your targets or enables it at all in some cases (e.g. limited wish + scrying). Plus, no dragon would actually want to burn all those scrolls if it didn't have to. Making a dragon (or any NPC, but especially a dragon) just happen to have on hand the exact right combination of scrolls to cast in sequence in order to obliterate a 20th level wizard, and making him actually expend those resources is pretty unrealistic.
And what do you really lose out of gaining those caster levels? They're pretty much the only thing you should be optimizing for anyway when choosing the dragon, why not make them high? Very Old Brass Dragon Sorcerer 1 is the same CR as the blue with 1 sorcerer level, and has 2 more caster levels.