One other vital point not mentioned is rehydration temperature. While each strain and manufacturer has different guidelines, they are all in a range of 80-105F, 25-40C, with most being at the top end of that range - much hotter than typical wort temperatures.
Rehydrating at room temperature can cause a loss of 60% viability compared to rehydrating at warmer temperatures. Why rehydrate?, An interview with Dr Clayton Cone, discusses how the temperature of the water affects viability and explains why rehydrating in wort (at any temperature) causes cells to die.
For examples of rehydration procedures, see
While you can get away without rehydrating yeast, you'll be vastly underpitching. You'll still get beer at the end, but it won't be as good.
As to why the beer won't be as good - Wyeast says this on underpitching:
Effect of Pitch Rate on Beer Flavor
Pitch rates, in addition to strain, temperature, and gravity, make a dramatic
difference in the final flavor and aroma profile of any beer.
The pitch rate will have a direct effect on the amount of cell growth during a fermentation.
Cell growth decreases as pitch rates increase. Ester production is
directly related to yeast growth as are most other flavor and aroma
A low pitch rate can lead to:
Excess levels of diacetyl
Increase in higher/fusel alcohol formation
Increase in ester formation
Increase in volatile sulfur compounds
High terminal gravities
Increased risk of infection
In a nutshell, underpitching stresses the yeast, since they spend a good deal longer propagating, in high gear, rather than fermenting. The increased propagation levels causes the off flavors mentioned above, and as there is initially less yeast, there is more chance of other microbial contaminants getting a foothold. High propagation levels leave lots of scar tissue from budding on the older generations, which reduces the rate of transfer in and out of the cell, making it harder for the cell to consume nutrients and expel waste products, reducing the effectiveness of the cell, contributing to high finishing gravities and stuck fermentations.
In the fermemntor, you want your yeast to spend their energy making alcohol, not babies!