We get better, they get worse - Failing to poach means that our new hires will be unfamiliar with our industry and our problems. It also means that it is a zero sum game. When we gain a great hire from a competitor the other firm will simultaneously lose a great employee. And being seen as weak almost assures that others will poach from you. Being a strong poacher might actually discourage others from taking your employees and if they do take them, strong poaching helps to make the win loss ratio closer to neutral.
Some strategic partners complain vigorously when their talent is poached.
Customers - Poaching employees from wholesale customers and suppliers can be a risky business if you do enough poaching so that they notice. Recruiters and managers need to remain vigilant to ensure that great recruiting doesn't cost us a great customer. It's hard work - Poaching, although necessary if you are to remain competitive, requires sophisticated skills and tools. Poaching is analogous to convincing a happily married person to leave their current spouse for another one across tandem. While single people might readily except almost any offer poaching happy employee individual requires a sophisticated approach if you expect them to even listen your offer.