Because the words they speak are an insight into the policy of their candidate husbands. Just like hearing the VP candidates talk, or the other party endorsers (i.e. Christie).
I recall a lot of people getting a giant hard on for Clint Eastwood's wacky words last week. I think the first lady candidates' words are are least marginally more relevant than a semi-retired Hollywood figure's.
Don't make any sense. Please. It's not the OSA way to use logic when emotion will do.