I also find it occasionally annoying if someone random just jumps in the middle of my fight, but this is an MMO. There is no such thing as "playing alone." We are all sharing the same space at the same time.
Furthermore, until the day arrives when boss-level enemies drop the class-specific gear we need on the first try and item farming goes away, I would prefer to keep the ability to jump into any battle I see fit.
I agree with thebigchin12.
By not allowing others to join battle, you are opening a can of worms where item farmers will camp spawns and keep engaging enemies until he gets the item drops he wants.
As we all know, that can take anywhere from a few minutes to hours.
In the meantime, you'll have a line of other players waiting for their chance to engage the same spawn in order to merely complete a quest or something and go on their merry way.
Battles need to be kept open in order to ensure fairness because sadly we really can't be counted on to police ourselves. If you are counting on another player to always be courteous, you may be hoping in vain.
to the people saying "if you dont want people to join, dont play and MMO", people like this game, but they dont like that random players join when the battle is so close to being finished. do you expect KI to make pirate101 single player? the vortexes are labeled "restricted area". why not let people do the same to all the other battles?
If you want to solo so much, why don't you just move your pirate to another realm where it isn't so crowded?
Then you can wander around by yourself to your heart's content.
I know I get super annoyed when someone joins the combat when you don't want to. Sometimes i'm not always in the best mood ( shocker, I know) and I just feel like being left alone and then out of no where some player hops into my battle and I can't do anything about it. I don't even have the option to say anything, because I only have Menu Chat! I like, however, that this game despawns an enemy when another player leaves. It would be wonderful if we could choose who can and who can't join! Also a tip for those of us who don't want others to join; go to the quietest area in the quietest realm, and there's far less likely a ratio for someone unwanted to join. Just go to the Options section and click on Switch Realms.
As vonawesome1 says on Wizard101, that's my two cents!