Why RPN?
Because people learn incorrect arithmetic from many calculators. If you don't believe me, go and ask a primary school teacher what 1+2*3 is, and see what proportion get the answer right. The clued up ones will ask where the brackets are, to which you should reply "none, just the normal laws of arithmetic".
When they didn't believe the answer, I used to get out two Casio calculators, one of which gave the correct answer, one didn't.
Besides, it is more intellectually satisfying to gather everything you need, and then do something with them. Gathering half, starting to do something, stopping to get the rest of it, then signalling that it can continue is ... messy.