This past weekend, I racked a 1054 OG stout off a yeast cake of Wyeast 1056 and put the airlock back on the carboy, brewed an 1122 OG barleywine and pitched the chilled wort directly on the old yeast cake. I used Jamil's recipe, and note that Mr. Malty himself recommend this approach in his podcast regarding barleywines. I think most would agree that you would be overpitching if you aren't moving from a low-medium gravity beer to a high gravity beer as I did.
I'm not rich by any means, but I wouldn't risk $50 worth of ingredients to save $6 on a new smack pack if there were any risk to using this method. The only reason I did it is because I knew I needed a huge starter for this barleywine, and I'm not sure how to do that. I think I would have stepped up a 2L starter into a gallon jug, then stepped it up again into two separate gallon jugs. Then I would have to decant to get rid of all that wort, and then bring the yeast back up to pitching temp. That seems like a lot of work and introduces other types of risk into the process, and I still would not get the huge 5 gallon starter I ended up with. In my opinion, this is the perfect situation in which to pitch directly onto a yeast cake. In most any other situation, I would just buy a fresh smack pack and make a normal starter.