1. I think this makes it far too easy to learn new cantrips. The feat that gives you 2 bonus cantrips, not to mention the elf and gnome racial abilities, are practically worthless now (at least for mages). Since cantrips aren't limited by either spell slots or preparation, I don't think mages should be able to learn an unlimited number of them. They should either make them spells that you prepare, or they should only let you learn a fixed number of them, IMO.
Exactly. In addition:
There has always been a weird mini-game for mages (wizards, M-Us) involving the spellbook. Clerics (e.g.) can draw from their list freely, but Mages need to have a spell in their spellbook. And so there is a mini-game of the wizard building the spellbook (assuming the DM cares) and making and hiding a backup; the DM maybe threatening the book, whatever. Mages need to look for spells, invest gold and time copying things out. If played RAW, it means levelling up on a long campaign is tougher for mages uniquely.
It may be some for some players, but it always marks the wizard as "other" -- and so the minigame often (IME) does not get played; it's a distraction for one, and not fun for the table.
What is the justification for its existence? Only the superior strength and versatility of the spells on the spellcaster's list (which some would question): it's a balancing mechanism, one that is often ignored. (Additional problems emerge with classes like the 3.x sorcerer, which lacks the book but casts arcane spells, and therefore has a hard limit on spells absent from wizards).
So, this answer has a number of undesirable effects:
a. undermines race and feat powers
b. gives mages access to all cantrips without mechanical cost (there may be an in-game cost for those playing the minigame), something that's not available to other zero-level-spell-users.
c. removes one aspect of the balancing mechanism without a commensurate decrease in effectiveness of other spells in the list.