I do think it is rude.
I have 3 children ages 5-5-6. Sadly, my 6 year old has had to stay home from two parties this year b/c my dh was away for work and I had nobody to watch the younger ones. He (DS6) is not old enough to just "drop off" yet. Maybe next year.
I would NEVER ask if I could bring 2 more children....what if everyone did that?
My good friend had a party a few weeks ago. She invited her son's entire class. This included my son, aged 6. My five year old twins were not invited b/c that would have opened the door to many, many younger family members of the birthday boy. This way she could say--"I'm sorry, I just invited the class" to cousins that weren't invited.
It was a good plan.
Would you believe THREE of the classmate's mothers brought younger siblings

Remember, younger family members of the birthday boy weren't even invited (due to the house already busting with just the classmates), now total strangers were bringing their children. Ofcourse, around here, mom's stay with their child. I think it is an unspoken and expected rule. Next year, I think parties will be just a "drop off" thing as far as DS6 is concerned. This should cut down on the Sibling Invasions.
And I think calling and asking if you can bring univited guests REALLY puts the hostess in an uncomfortable position. Just b/c they feel badly and say yes, doesn't mean it is right.
As always, just my opinion.