Halon is a chlorofluorocarbon (CFC) and is considered to be therefore a bad thing that should be replaced with other less environmentally hostile substitutes. I'm not an environmental activist but do on the whole consider the Ozone layer a good thing.

The reason that it is used in placed like computer system (rather than the more normal carbon dioxide used for electrical fires) is that carbon dioxide can scramble magnetic information (such as tapes and disks).