The biggest problem with XP boosts applied to the leveling system that exists in the earlier years of wow is that your character quickly out levels their ability to obtain level appropriate gear and collect enough gold to buy their skills.
The gear rewards from quests aren’t enough to keep up and at some point (somewhere around lvl 30) you’re walking around in lvl 14 gear and have a very hard time killing even level mobs, especially if you’re a melee class and have to get beaten on. Typically the quest rewards are augmented by dungeon loot and items purchased on the AH. However, when talking about 1-58 post TBC content release, the number of people to run dungeons with will be scarce as will the availability of low level items on the AH.
On the gold front, even in pre-tbc leveling changes most players have a hard time obtaining enough gold to purchase their class skills. This is compounded with the need to buy a mount at 40 (lowered to lvl 30 and reduced in price by 1/2 in tbc) leaving the character unable to purchase gear, skills, and mount at the same time.
Both of these things intermingle to provide an overall crappy experience for new players that don’t have a patron main to fund their leveling activities. This is made even worse if that player also wants to try to improve their professions while leveling.
The instant boost solves these issues by auto training level appropriate skills, providing a slow mount, and a set of crappy but level appropriate set of gear. This allows the player to at least be functional after the boost where as the XP+++ player is stuck in a never ending loop of difficulties.