I've been using yeast from Fermentis for a few months now and I have really enjoyed the resultant beers that they've produced. Not only have the beers been quite nice, the cost of using dry yeast is lower than using liquid yeast. Additionally, it does not require a starter. Should you need a higher cell count for a higher gravity wort, simply pitch more. The cost, at the home level, is sufficiently low to be able to do so.
Today we're going to discuss two aspects of dry yeast: Glycogen Reserves and Rehydration.
The first topic will be Glycogen reserves. Glycogen is for animals what Amylopectin is for plants. In yeast, glycogen reserves serve to maintain life up until the point that glucose uptake from wort is possible. Producers of dry yeast (e.g. Danstar, Fermentis, etc.) take special care to make sure that their yeasts have adequate glycogen reserves to improve their survival rates. Interestingly, this is why it is often not advised that one make a starter with dry yeast. Making a starter uses up the glycogen reserves that the producer had included.
Rehydration is also somewhat related. If you've ever pitched dry yeast straight into a wort without rehydrating beforehand, you probably haven't encountered any problems. But if your beers are turning out fine, why bother with this mysterious hydration business?
In the first few seconds that your yeast is pitched into its new environment, its cell walls are highly permeable. Sugars, hop derivatives, and a host of other wort constituents are capable of killing the yeast at this point as there is no mechanism in place to filter out harmful compounds. Water is the preferred environment for conducting rehydration. See here.
So why is it that your fermentations are proceeding just fine sans rehydration? Easy. The sheer number of cells available in modern dry yeast is quite high. Apparently high enough that the deleterious effects of pitching without rehydrating doesn't seem to cause a huge issue. However, given the small amount of effort necessary to properly rehydrate yeast, there doesn't seem to be reason enough not to. Of course, you could just increase your pitching rate, but don't think that having a bunch of dead yeast in your fermenter is doing wonders for your beer.
By the by, there are some succinct rehydration instructions available here.