Yes, a powder ski is a great thing to have ................. WHEN YOU HAVE POWDER. If you ski in NE, powder skiing represents, on average, about 5 to 7% of one's total skiing with a 3% margin of error. Unless you are wealthy or a shop rat gear junky, you are probably not going to drop $1200 on powder boards and bindings. Most New Englanders probably ski on an all mountain ski. A second pair is most likely going to be a carver or race ski. A third pair is likely going to another race ski or bump ski. A powder ski in NE should probably come in at the fourth or fifth ski in one's list. That said, the average NE skier only skis with one or two pairs throughout a season. Again, I absolutely agree that a powder ski is the best thing to have for powder and, in light of the above school of thought, renting a pair for the 3 to 5 days per season is probably the best option.
Although, due to successful marketing campaigns we are seeing a few more powder skis regardless of the mathematical logistics stated above. What this results in is that we see a lot of skiers skiing on ice, bumps and groomers on powder skis. This is a sad sight to see as it is a very poor option in terms of ability development. This is a significant aspect as skiing is a sport that is highly developmental in nature. Skiing with fat skis on groomers is the equivalent of riding a bicycle with training wheels. On the steeps, bumps and ice, more often than not, this will produce awkward movement patterns. These awkward and poorly derived movement patterns become ingrained to the point that can make it difficult to go back to a ski that requires much more balance, the result of which, can "developmentally" trap a skier on a ski that is only good for 5 to 7% of the terrain they will encounter.
On the other side of the coin or, the other side of the country, I believe that, on average, a powder ski would move up to the second slot in one's quiver. However, due to the above mentioned successful marketing and the ease of use of a fat rockered ski, for many out west, a powder ski is the only ski they will ski on throughout the season. This, again, is a detractor of the types of developmental opportunities only found on a skinny ski.
This is only my personal opinion, the logistic rational of which, will not translate to those who love their powder skis and rather not delve into any details that would kill their buzz. Completely understandable.