[packetslave]

You'd be amazed how often "standard" network protocols behave subtly different between vendors. You have to exhaustively test interoperability for every single feature and config option if you want assurance that it isn't going to break in some bizarre way.

[rdl]

It is also really nice to be able to call one TAC and have them devote effort to fixing it. If you have a heterogenous network, they can pass the buck, or even if they are awesome and try to help you out, there is no way Cisco's TAC knows as much about Juniper stuff as they do about Cisco, it is harder for them to put together a duplicate config, etc.

Back around 2000 this was a big deal. Cisco slacked on gigabit routers, and Juniper didn't have a comprehensive product portfolio, so while SP networks could be all J (but maybe with some switches from Extreme, etc,), enterprise networks were a lot more likely to have juniper and Cisco mixed, if they needed juniper performance in the core. Juniper ended up broadening their portfolio and Cisco improved their high performance offerings a few years later.