There are two main forms of alternate scoring cards. The first changes the win conditions, making it overlap with Rule Card. The second can be either interesting or useless, depending on several factors.
This second kind is difficult to explain. It works by giving or taking away different kinds of points; not just normal points. If these different points are well-established, cards using them can be very useful. If not, they're just burdensome to your Scoresheet.
Often, creators of these different varieties of points will be trying to establish them as commonplace as quickly as possible.
Common alternate scoring:
-Magic points, usually to restrict the usage of powerful cards.
-A form of money. Often, there are "store" cards which use this money in return for points or extra cards.
-Often, there will be things such as "weasel points" and "onion points" that exist only for humor value, with no intention of doing anything useful.
-Some may, of course, do other things.