Planning in doing this at the end of the month (from an APA to a stout). Since that's my plan, I've done a bit of research over the past two weeks. I have heard essentially what others have said above, but it all sounds like opinion and anecdote. I'm an engineer and I like facts and evidence.
The one thing I have real evidence of is a quick (really!) experiment where someone tried it. Because there is so much yeast, the yeast is at its prime, and there is a ton of fermentables, fermentation takes off extremely fast. What I saw was a time-lapsed video showing the blow off tube getting filled in under two hours.
Since the search function on HBT is no google, I suggest you google "pitching onto a yeast cake". Search through the hits that link to HBT. you should be able to find the video I saw.
So in the end, it is your choice what to do. Make your own decisions. But use a blowoff tube and maybe have a swamp cooler ready, because that fast of a fermentation WILL kick off a bit of heat. Good luck and let us know how it turns out.