Fact that you can copy magstripe keycard has no security implications. The whole idea behind hotel keycards is that each guest gets unique code on keycard and thus cannot use his copy after leaving hotel. In fact it is pretty secure design: you don't defend against copying, you simply expect that copying is possible.
Except it is easy to fake the key, there are videos of Major Malfunction changing the key to lock out all other keys, to lock out "older" keys and all kinds of other neat tricks.
Copying and modifying the data is too simple, there is no real security on those cards. The data is not encrypted on the card, it is just a proprietary format. Once you get a couple of cards it becomes easy to understand the various different data fields and what they mean.
