dave2008
Legend
I don't have such worries. I concern myself with making the game fun, not meta concerns of statblocks or terrain or encounters or... I do realize everyone is different and as different issues.The reason I give my spellcasters the full complement of spells is for the same reason I memorize all my PC spellcaster spells carefully. Because I want to maximize my chances of having something useful to do in unknown future situations.
If the PCs do something unexpected or outside of the narrow spells shown in the OP, I'm put in a bad position. I know that Vecna would have many more spells than he is given, so what do I do? If I just give him the spell needed, then it just feels wrong to me because he didn't have that spell in advance, so it feels to me like I'm giving him what he needs to get out of jail free. On the flip side, he's a freaking spellcasting genius and would have a myriad of spells to get out of bad situations, so if I don't give it to him, I feel like I'm shortchanging him.
I don't want to be placed in a position where both giving him the spell he needs and not giving him the spell he needs feels wrong. If I give him a full list, then if he has the spell I can use it with a free conscience. And if he doesn't have the spell, I don't feel bad about leaving him in the bad situation. It's a lot more work and most of the spells won't be used, but it's worth the effort to me to not be placed in a lose/lose situation.
I gave my Vecna (Vecna the Archlich) a full slate of spellcasting options as I like to design / create monsters that speak to the whole world. Such detail is really just a design exercise, it is not really useful for me. Actually, I don't usually use the monsters I make and prefer just using MM versions and improvising anything extra I need when we play.