lol i'm not sure where you got that idea whtat most games today do not require a disc to play but they definitely do
Anyway, i would imagine it being very legal, and there are ways to do that like mini-images. They're not the game itself so i would think their distribution is not illegal, that and there are always cracked exes that remove the requirement of a disc.
Whether its legal or not really depends on perspective but to me as long as you own the original, and you don't wanna scratch up your original disc for whatever reason, i would think its perfectly fine.
Regarding playing the games online though, thats a bit more difficult as some servers actually try and detect that like how i think Battlefield 2 did it. They even banned in some cases if you ran cracked exes.
However, with mini-images, there would be no real way for the game to detect without collecting information on the computer and sending it out without your consent (which is of course a breach in privacy policy) for them to detect you're running a mini-image.