Putting patches on rips and tears in your pants seems like a no-brainer. That is, after all, what patches were originally used for. However, you don't really get any option as to where to put those patches, so you might end up with one on the back of your ankle or knee, and no one will really see it there. That's why people usually use really thin, dull patches to repair rips in their pants. However, you can use decorative patches on your pants if you want, especially if you're using them for decoration and not for repairs.
The biggest question is where to put these decorative patches. It's difficult to see patches on your pants. While your jacket is usually very visible, your pants aren't always. This is especially true while you're sitting down. Any patches put on the back of your pants won't be seen. Patches on your lower legs might not be too visible, either.
Space is another issue. The largest part of your pants is the back side, of course, but if you put patches there, be ready to have people staring at your butt all day! Patches on the back pockets do look very nice, however.
Another problem is that some decorative patches are thick and don't bend very easily. If you need a patch on your knee, it's going to have to be flexible. Some decorative patches just won't bend quite right and can't really be used on the knee.
Adding decorative patches to your pants can be done, but you have to make sure they're small enough to fit where you want them to and flexible enough to bend as needed. You also have to be aware that they won't be as easy to read or as visible as those put on your jacket.
