Spellcasters are definitely important for villains. Whether the villain is a spellcaster, or has spellcasting henchman.
Actually, a villain really should have a blend of henchman, much like a party. You don't want to be deficient in some basic way. You want someone with a good Spot, to notice rogues sneaking up on you. Give that someone See Invisibility.
You also want arcane magic, for various important effects and countereffects.
You want divine magic, so you and your henchman can be healed quickly if need be. And to turn/control undead that might pop up. Freedom of Movement can be very useful against Held or Entangle, and Restoration is vital for long term survival.
You want fighter support, to at least beat up villagers, if not fence off foes from dogpiling the spellcasters. Fighters who can fly, at higher levels.
Villains should be quite willing to withdraw. Strongholds protected by permanent Mordenkainen's Private Sanctum are vital at higher levels, but you need an arcane caster of 13th level or more to do it.
Other scry-proofing, like Nondetection, Mislead, or, the ultimate, Mind Blank is very handy.
Cool thing for evil wizards:
Contingency spell - teleport. Contingency: 'If I attempt a teleport and someone interrupts it, teleport me to Safehouse 3.'
Perfect for those 'Aaaarggh! PCs are swarming over me, I can't cast my spells!'
Anyway. Some random points about villains.
