I have seen several products ( such as the pitft for the raspberry pi, and most other small tft screens) which use a 3.3V regulator on the 5V power rail to provide power to the 3.3V system. In some of these products, given that they are breakout boards for systems which have a 3.3V power rail which can supply the needed current, it seems that the extra 3.3V regulator could be omitted. Is there any special reason why it might be beneficial to include this external 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.