Yes it would be nice if PI could do that. The problem is that once you add simple features people will want more and before you know it you're writing a whole new image editing program.
For those of us who use Windows but don't have Photoshop I can recommend PaintShopPro X2 which is far cheaper (look for OEM disks) but can edit 16 bit TIFF files. On linux there's CinePaint which is a GIMP derivative that also supports 16 bit images.
Besides, many masks work fine in 8 bit depth and then there are far more options.