If I'm just harvesting nodes for my botnet, macs are pretty lousy targets, no more capable than PCs and substantially more obscure.
If I'm attacking systems for the data on them, or to MiTM/trojan/keylog the users of the systems; grab banking credentials and the like; mac users are a conveniently self-selected group of people atypically worth harvesting. Sure, there are a bunch of underemployed baristas with degrees in Individuality using the macbook pro that mommy and daddy bought them to watch movies in their dorm room; but as a whole, thanks to the higher prices, users of OSX devices skew upmarket pretty substantially(iOS devices have some of the same effect; but much less, since at least an iPhone 5c or the like is probably available as the 'free'-with-usurious-contract model on most telcos).
If you are attempting a corporate/institutional intrusion, macs vary in value: they are way, way, less common, frequently absent entirely; but where they are present, their minority status often means very limited integration into the enterprise's legion of 'security' products, IDSes, and everything else that the Windows users complain is causing logins to take 30 minutes. This makes them handy 'beachhead' systems, especially if they are loaded up with Office, Adobe Malware Runtime, and similar stuff that may well have cross-platform or partially shared libraries of vulnerabilities; but much reduced vigilance on OSX clients.