I'm a PADI rescue diver not an instructor so I'm not sure if it's my opinion that you want, but I think that, if done properly, this Doff Dawn thing is a good exercise. Every class i've ever taken or seen has had to work on controlled swimming ascents w/o air, and gear removal/replacement under water. This seems to be a culmination of the two. However, it does seem strange that they are asking you to do the ascent without your tank. If you are talking about a depth of 10 feet or less, then it seem like it would be safe, but a relatively long ascent, like 20 or 30 feet, should be done with the tank just in case theres a problem. (in my last class, I was told to do the out of air ascent with my reg in my mouth even though I wasn't using it. Hope this helps.