I guess it depends on the size of the portlight, how strong it is built, whether you are concerned it might warp and leak, whether you are concerned you might be caught in a storm, your tolerance to risk, how much you like living, willingness to spend money on strong frames to avoid leaks and stay alive, or prefer a cheap solution, whether you are prepared to do work to make a portlight fit the existing cutout or prefer to get them custom made to fit, etc. There was some discussion earlier:
www.seabreeze.com.au/forums/Sailing/General/Opening-ports