The most important concern is hygiene. If there are people other than you using the same toilet, there are chances for the toilet bowl to catch urine splashes. Even if that is not going to be an issue, it is not really comfortable to sit on toilet bowls. They tend to be narrow, hard, and cold. As the edges of the bowls are narrower, your thighs will hurt because of the pressure. You might not be able to sit like that for more than a couple of minutes. Also, toilet bowls are uncomfortably cold and sometimes wet. Therefore, it is better to have a barrier between the bowl and yourself for comfort and hygiene.