The only real reason to get a vga adaptor for anything is to increase the quality. However, generic vga adaptors are converting the video to a higher sync so your monitor can display it... without killing itself trying to do so (vga runs on rgb, and most consoles output rgb natively, just at a sync meant for your tv instead of a sync meant for your monitor (as it is... progressive scan games that are output in rgb supposedly can be connected directly to a monitor)). Now any time you do a video conversion it lowers the quality, even if you are converting to a better style of video... the quality lowers atleast to a degree. As such what you are talking about doing is displaying something that's sub-composite or sub-svideo on a really nice screen. Now no matter how good the screen you have is... if the video isn't good, it's not going to be made better by going to a better screen (your monitor). Short of one of those xrgb devices that cost an arm in a leg, it's not really worth getting a generic vga adaptor, atleast to me... cause I don't wanna pay $50ish dollars for lower video quality than I'll get on a normal tv lol.
(as a side note, the dreamcast can natively output vga, so it's vga box works rather nicely. The only ps2 vga kit worth using is the one that only works with the linux kit, and I'm not sure if that's actually vga (doesn't seem likely) or if it's converted video (probably). The xbox supposedly can output vga natively too if you patch the bios. No other systems I know of can output vga natively.)