This is the small thinking that is destroying the mmorpg. The “everything must be on the bars, and be useful most of the time” mindset is catering to the adrenaline junkies that WoW catered to so they could dominate the industry. These are the people that would jump ship as soon as things a little less exciting, leaving little for those that would commit to a longer, deeper game nothing of substance.
The Archetypical "Wizard" is a seeker and keeper of knowledge of all things arcane. They would know thousands of spells, most of which would serve no purpose in combat, but have other uses. Many would be situational. They would have spells they choose to use in combat that set them apart from other wizards; a personal flare that fuels the stories about them.
Similarly a Martial artist often learns multiple disciplines, at least so they know how to counter them, but may pull from them in certain situations.
Not everyone wants to play like that, but allowing it, gives people more options. They could choose to focus on one area or branch out.