yeah mixed brand cameras has a few advantages, but given the wildly different configurations, features, compatibility issues, etc.. going with a fixed brand seems to have more advantages.. You can keep spares about (even use them in non-critical locations until needed elsewhere), learn a standard config and how to best tune the cameras easier, use the same software to view all the cameras directly and having a common firmware with identical settings and capabilities makes for less weird issues and compatibility problems.. You can also get a few of the same cameras with different lenses/fov and test them out in actual locations before committing to the rest of the cameras.. this way you know exactly what lens you want where and order accordingly.. You can also roll out firmware updates slowly testing for problems instead of finding killer bugs after the fact.
Having a network thats built around common devices is a smart design decision from an administration standpoint; nothing is worse than a network with different brand switches/routers/modems/access points everywhere because its a nightmare to maintain.. If end users have access to the cameras this is even more so; they will bitch and complain if they have to use different browsers and everything is not uniform.
Anyone looking for a IPCamera setup with more than 3-4 cameras should seriously consider a homogeneous setup, and research the brand you lock your self into prior to committing a bunch of money to it.. Few will find the advantages of a Mixed brand camera setup will exceed the disadvantages beyond this point.