Yes, a RSVP is meant to allow the hosts to get an accurate count, but I do think there's a difference between a formal catered meal where the caterer may be preparing exact numbers of items and where your coming or not coming has a significant financial impact on the hosts, and a more informal meal such as a BBQ, where hosts don't want to prepare enough food for 60 when only 30 are coming (or vice versa!) but don't really care whether it's 53 people or 57 people coming because they'll prepare about the same amount of food. DH and I throw an Oktoberfest each year. We don't ask for RSVP's (though we get a lot of them!) because we know approximately how many people are coming and approximately how much food we'll need (we don't ration people to one sausage apiece!).
You can decide to go to Janet's party if it's held on the original date and go to Katie's the next day, missing Janet's if she has it on the rain date (which may not be fair to Janet).
You can go to Janet's party whichever date it's on, and tell Katie you can't make it.
You can tell Katie what the circumstances are, that you can 't commit but don't want to put her in a difficult position, and see what she says. She may say, no problem; come if you can; or she may say, I'm making creme brulee and need to know EXACTLY how many to make.