After another re-watch I've finally gotten why it was so significant.
Borden says to Angier that he doesn't know what he has been through to be a magician. How he has sacrificed so much. Angier responds with something like
" I have known sacrifice, I have..."
Borden just tells him to shut up. He refuses to acknowledge it which ties quite nicely with Cutter's voiceover saying:
"You don't want to know, you want to be...
The "fooled" is said at the same time a clone is shown in the bath.
This was done to show Borden didn't want to know what atrocities Angier did to try and defeat Bordon. Angier wanted to finish thier rivalry once and for all, and he refused to continue it. In the same way the audience would refuse to continue trying to see the trick's answer.
It's very clever.