I feel mind-blowingly pedantic saying this, but phrases are rude or acceptable solely based on the consensus of the people using them. If "can I get a..." makes you nuts, you are outside the community of people who find it acceptable. Likewise, if it doesn't bother you, you are inside the "can I get a..." community. There is no right answer or moral dimension to this (and I'm someone who usually believes in moral dimensions and right answers), it's just which group is larger (or higher status or...whatever criteria you want to apply).
Honestly, we blow air across our vocal cords and waggle our tongues and flap our lips and these noises come out. What they mean is a collaborative exercise.
There. I feel better.