Yes, I would find that rather rude.
BTW, You can get SB's inside the park.
I totally get this and it sometimes annoys me when I see it happen especially if it's a large party -- that doesn't seem fair. However, a party of two or three doesn't feel like a big deal to me and I see it happen all the time. Accordingly, I simply let it go. Everyone seems to do it and I've even done it myself. Moreover, when this happens and I'm still the first on PP (and I almost always am), I feel that there's no real harm in it. So two or three people got ahead of me by joining their party. I still got what I arrived early for -- first in line on PP.In my opinion having a "place holder" is not cool.
Do it. It's your vacation what others think doesn't matter. Might you tick off the person behind you? Yep. Ya might. Odds say they won't say anything and therefore you won't have an issue. If ticking someone off matters enough to drag the whole family out of bed a half hour earlier to you that's your business & I don't judge. Personally, the person behind me can just deal with it. After all, when is the last time a fight was reported anywhere over this? Never. (The person behind you is stuck because if they say something & it starts a fight you'll both be pulled from the line.)
To each there own. I hope that's what OP gets out of the discussion.You see a lot of this attitude at the parks, and pretty much everywhere in life, it's rude and it's selfish but it's there and you are right, people don't usually push the issue.
I like that the OP was trying to ask and sniff out what is considered rude behavior and trying to be considerate.