03-09-2012 05:37 PM
Pretty much what excelence said, with a little bit of elaboration:
Generally, unless you do it a lot against the same person, people aren't READY for rapid cancels. There are circumstances where good players might be (They see you have 50 heat and they just blocked something that'll lead into a big punish - Inferno Divider, or Magnatech Wheel or something) but in general, RCs can be quite surprising, and can lead to people trying to punish you when, it fact, you are at frame advantage. Doing something like Ice Car -> RC -> j.B with Jin can result in your j.B hitting someone who was getting ready to punish your Ice Car. This can lead to all sorts of mixups.
The only downside of RCs is using the heat for that instead of for something else, though for many cast members, Rapid Cancels are one of the best uses of heat.