I have never heard that BS about making a frame brittle. And there have been many folks here that have had their frames done, me included. If Pete Lohr, an engineer in this stuff had his done, you can take it to the bank it's safe. Think about it. If HDG (hot dipped galvanizing) tempered iron, every hot dipped galvanized nail would break instead of bend. Besides, the frame is only in the molten zinc bath for maybe 30 secs. It would take more heat longer to harden the frame.
As for weight, it adds less than 75lbs to the weight of the frame. About the same as driving around with a small child in the back seat. If you're a racer, you want to trim every ounce possible so you wouldn't want to do this. However, if you're looking for the best and cheapest way to de-rust and protect the frame of your cruiser beyond your lifetime, get it galvanized. It's done all the time.