That and the fact that clinching does show up when Boxers or Muay Thai guys fight, and pummeling does show up when wrestlers compete. When was the last time you saw anything that looked like Chi Sau show up in a sparring match??? And I know! I know! The response is going to be "Chi Sau is not fighting. Chi Sau is for developing attributes used in fighting. Why would you expect fighting to look like Chi Sau?" My answer......people have tried to make a direct comparison between Chi Sau and things done in other fighting systems. However, as NI suggests, the platform used for clinch work is the clinch! And the clinch is something that happens in Boxing, Kickboxing, and MMA. The platform used for pummeling is the "tie up" position, which is something that happens in grappling. The rolling Poon Sau platform for Chi Sau is very artificial and doesn't show up in sparring. So it is not really directly comparable to clinch-work or pummeling as suggested.
However, it can be trained more realistically. DanT's school seems to do so. I know Rick Spain's guys in Australia do so. And using the Poon Sau rolling platform as a transition to these close-in grappling kinds of applications doesn't take a "high level" training in playing the Chi Sau game that so many work on. It just serves as a good "jumping off point" when contact is made with the opponent.
Another factor to consider, you can take it or leave it, is that JKD schools have never emphasized Chi Sau to the extent that Wing Chun schools do, and many of the JKD schools today don't seem to do much of it at all. Some have dropped it completely. Why is that? Because using a practical mindset they came to the conclusion that it isn't a high yield exercise to spend time on when it comes to being able to fight effectively. And every JKD school I have seen seems to spend far more time on actually sparring than on doing Chi Sau. Take that for what it's worth. Just another data point in the discussion.
I can't speak for any one else, but we apply the techniques we practice in both chi sao and two person drills into our sparring, such as lap da, tan da, gum da, etc. We use chi sao as a platform to build timing, speed, power, and proper execution of the technique. The poon sau is not there in sparring, but the moment you bridge, our chi sao techniques and reactions come into play.
Also our chi sao doesnt look like a lot of schools. We roll and excecute a single technique to pin or move to the outside while striking, and then use one or two follow up techniques to continue the pin and strike, or move in and work on the clinch to control. We don't usually continue after two or three exchanges of techniques because for us, the primary purpose is to make the first technique and the follow up perfect.
The poon sao for us also focuses more on developing both the heun sao skill (as we usually switch sides every two or three rolls), and to me the heun sao is a very effective combat technique.
I find in too many schools the emphasis is placed on poon sao, but in my school, the emphasis is on the execution of the first and second technique, and the switching with heun sao, and the clinch work with neck control, elbows, knees etc).
At least thats what Im FORCED to work on, because I am 5 9' 170 LBS, while most of my training partners are over 6 2', 210 LBS, and are all mostly muscle. I love chi saoing people who are stronger than me tho, as it forces me to have perfect technique and timing.
But OK by me. 
