It goes without saying that stamp collectors could not do without their catalogs.
Catalogs should be recognized for what they are -- guides, not the final word on the value of stamps. Unless, like Stanley Gibbons Ltd. catalogs, the publisher is also a stamp retailer. It's great for collectors who quote high catalog value of their stamp collections to non-collectors -- the value impresses, while the collection's owner knows that he spent significantly less than the figure from the catalog he bandies about.
Sometimes catalogs are overtaken by events. Imagine catalog publishers trying to keep pace with the rocketing values of the stamps of China and India, two current stars of the stamp collecting world, thanks to ever-increasing demand.
A casual look at asking prices (eBay is a good casual barometer of stamp value) will show that there is a very large discrepancy between catalog value and realizations.
Sentiment Affects Stamps' Values
There are many factors impacting a stamp's value. One of those that the catalogs can't gauge until the market sets the tone, is sentiment. The values based on sentiment can be a mercurial thing. Think of the Princess Diana stamps and how they reacted to the attention cast on them by the tragedy of Lady Di's death. Prices of the Charles and Diana wedding stamps took a jump and then settled back into a more reasonable price range.
More recently, there was the blip of interest for the 50th anniversary of John Glenn's space flight in the Friendship 7, an event commemorated on a 4¢ stamp issued upon Glenn's return to earth in 1962.
As regards the Glenn stamp, there is an aspect that catalogs do not record -- the impact of first day covers on a stamp's valuation. In a strict sense, considering the small number of first-day covers in relation to the entire stamp issue there shouldn't be a correlation.
Value of Stamps on Cover
While many catalogs, including the Scott Specialized Catalog of United States Stamps and Covers list a number of first-day covers issued what it cannot gauge is how the FDC interest impacts the value of mint stamps. Scott's Specialized will also give you a value for stamps used on cover up to 1940. These listings are, of course, beloved by collectors of postal history, as it gives them a guide for the value of their collections. The selling prices of these items are often closer to catalog value than the stamps themselves.
Today's Potential Cover Rarities
This should be a warning for those who think the collection of modern postal history is a dead end. Collecting current mail is a useless activity; it may turn out to be just the opposite. In fact, collectors may someday find themselves looking at rare items that are today just mail. After all, stamps are seldom used on mail: when they are it is almost an anomaly.
Some enterprising philatelic publisher could do well by producing a catalog showing the number of stamps used on the cover and offering their values - only as a guide of course. It is beginning to look, in 2012 as if these will be as rare - perhaps even rarer - than classic stamps that were issued in much fewer numbers than current stamps used on covers. Interestingly, stamp collection seems to have come full circle, and with rarities being created right under our noses.