The BusPirate is/was more aimed as a general purpose communications sniffer, although it's expanded to become a swiss-army-knife of embedded development. Likewise the open logic analyser which is also a bargain.
I would not say a BP is the best way to get firmware into an embedded micro for development purposes (a dedicated debugger is probably going to be best) but I would say that it's well worth having a BusPirate, LogicSniffer and if you can stretch to it, a DSO-Quad.
All three are incredibly useful for embedded development, all three have paid for themselves many times over here in time saved even though we have all the "proper" gear in the lab to turn to.
The BP we have found particularly useful when trying to get a new device (EEPROM, SPI periperal/sensor etc.) to talk to our micro as you can get the device up and running through the PC before translating the magic incantations into embedded code with some confidence that you're sending the right commands in the right order.
For embedded programming/debugging, a dedicated debugger (usually comes with the devkit a-la MSP430 launchpad) will probably get you going much quicker and intergrate with an IDE easily.