You will always be struggling against the saddle in dressage, if you ride in an allpurpose. It just makes it harder to keep your leg down and under you. You can do it, but I bet you can do it better in a dressage saddle.
If you are serious about your competing I think it makes a world of difference to have two saddles. If you're putting that much time and money into your events, it only makes sense to have gear that will give you the best advantage.