The only use I found for "Disguise Self" was changing the appearance of the ugly hirelings for aesthetic reasons... A lot of spells in the game just doesn't seem to have any real usefulness, even in areas where they should have
If you murder somebody and then cast speak with the dead on them to interrogate them they will often say that they are not willing to speak to their killer. Rarely, you can pass a persuasion check to talk to them anyway, but usually it fails. However, if you cast disguise self first then they will answer questions with gusto.
I'd forgotten that disguise self works with speaking to the dead. It's just odd that the game will have an NPC just start talking to you as if it knows you, when you are disguised as a completely different person.
I know that the game needs to interact with the main character from time to time, for the story plots. And if the game does not talk to you, it could cause real issues with parts of the game. But if the game really needs for particular NPCs to interact with you, to make the game work, the game could have a cut scene where the NPC approaches you, and starts the dialogue, but then says "Oh I'm sorry, I thought you were someone else," and then the games narrator can say something to the effect that it's advisable that you drop your disguise to see what this person has to say.
