If you are able to put out a good burst of power while seated, that's certainly a valuable ability; you can quickly close gaps, and I've found that it can be beneficial to attack from the saddle sometimes. It's a bit less conspicuous. You see a lot of guys, in the lower cats anyway, who get up out of the saddle and thrash the bike around a lot but don't make much headway. That's not what you want to do, whether attacking or sprinting. But that's a matter of technique, and once you've practiced to the point that you're not throwing away energy, you'll generally make your best sprinting power and speed out of the saddle. That may be the issue you're having.