Need for 3.3V regulator
Apart from too much current and heating issues when you connect to already existing regulator which supplies power to another board, there's a responsibility reason: if your board will fry existing regulator (in your case raspberry's one) whole configuration will stop working, and you may be held liable for failure of whole device. If your device will fry its own regulator, (hopefully) only your device will stop working. Thus while addition of part adds some cost, it is wise to have it on the board thus having predictable current consumption of its output, and protect other device from issues with your board.