Does Caitlyn ever actually get Master Yi's legend Might bonus, given her ability's speed restriction?

Caitlyn only gets Master Yi's legend Might bonus while she is actually defending — meaning she's on a battlefield your opponent is trying to conquer. However, her own ability is not Action or Reaction speed, so she can't be activated while she's in combat. In practice, this means the two effects rarely combine: whenever she could get the buff, she can't use her ability, and whenever she can use her ability, she isn't defending and doesn't get the buff.