I've never seen signed-integers being required. They tend to introduce unexpected bugs rather than prevent it. Of course you can accidentally end up with signed integers when you didn't intend it. Here's my favorite accidental undefined signed integer overflow, assuming 32-bit integer size (the 64-bit version is similar)

  uint32_t foo(uint8_t x) { return x << 24; }

Yes, this is signed integer overflow, since x gets upgraded to a signed integer before the shift, if an integer is 32-bits in size, this can result in undefined behavior if the top bit of X is set. Fortunately I've never seen a compiler optimize this to stupidity.