Well, people who want an excuse will use any excuse. Of course for people with very bad dissociation, sometimes child parts can take over and they don't even remember it (this is part of the diagnostic criteria), so it's more of an explanation than an excuse (although not everyone who claims to have a dissociative disorder actually does!) For the rest of us, thinking in terms of "parts" is just a way of understanding internal conflicts we can have, and shouldn't be an excuse for anything. It's no different than someone saying in casual conversation "part of me wants another slice of cake but part of me thinks I shouldn't."