What is the advantage of being a Math/CS double major in graduate admissions for someone who wants to work in TCS?
Studying mathematics is great, and I agree that many theoretical computer science students would benefit from studying more mathematics.  However, double majoring is definitely not a must, and it will help only slightly with admission.  It could serve as a tie-breaker, but it is less important than getting good grades and much less important than doing research.  Ultimately, a double major is worthwhile if you would enjoy it or find it useful, but the learning will have to be its own reward.