Sometimes they can be great fun, sometimes they ruin all of the fun. It happened more than once that a strong IR force attacked my NR station which only had a small and inferior defense force, making for a fun challenge to still try and win that battle with the few X-Wings, Y-Wings, Corvette's and that one Dreadnaught I had to make do with. Then out of nowhere, a few destroyers in pale green jump in and attack the IR forces, destroying them. Because the battle is not over, but the IR technically has 0 forces, they call the retreat long before that third party ever makes it to my station and a good battle is ruined.
However, there's nothing more fun than declaring a victory too early when your damaged SSD has cut a path through the enemy destroyers and out of nowhere a small group of third faction ships jump in to give your SSD a hard time, forcing you to call back your escorts to save your flagship which gives the original opponent of the battle the time they need to regroup some of their vessels for a stronger defense. That adds surprise and fun to the fight!