You wouldn't throw it. Because in real life, semtex isn't used as a sticky spherical timed explosive you can throw and have stick to people. In fact, semtex is used the same way C-4 is. Also, semtex in real life is orange. But as for MW2 context, I'm not sure how you would throw it. A partial sticky area would not be practical because you wouldn't have 100% successful sticks. The simple explanation: It's a game.