Kind of a waste if you ask me. At most, Signature Spells gives you two additional 3rd-level slots per encounter. It's not going to significantly affect combat at level 20.Yeah, starting a wizard from level 1 as a cleric/wizard is a good combo since it gives you guidance, armor, and basic healing. But if I was running a game starting at level 20 I think I would just take an abjurer with counterspell as a signature spell and make enemy casters weep.
Spell Mastery has way more impact than Signature Spells.