Good points, Leanan. It does seem like Venka's powers are perfectly suited to deal with Winnowill. And yet, Venka was conceived before either of her parents had met Winnowill! So there was no perceived need, there. And, of course, Pool was conceived before Scouter and Tyleet had yet met any of Winnowill's creatures. Which does lead, as Nowth said, to the conclusion that these powers are not coming about in an evolutionary sense, as an adaptation to the environment, but rather are being engineered from outside (above?) somehow.
Could be the High Ones' spirits are actually influencing the particiular gifts a magic-using child is born with. Timmain did say "interested spirits" were attracted to those with gifts.