From a functional standpoint, yes, you could essentially achieve the same effect with multiple stacked filters as a single high-density filter (say a 10-stopper.) There are a variety of concerns to be aware of, however, regarding stacking multiple filters.

1. Filter quality:
   • The Lee "Big Stopper" 10-stop ND filter is pretty high quality glass filter
   • There are two Lee ProGlass filters (0.6 and 0.9) of the same quality
   • The full range of standard Lee ND filters, (0.3, 0.45, 0.6, 0.75, 0.9) are resin
   • Many third-party filters are resin, a very few are glass (UPDATE: Since this answer was posted, many more third-party manufacturers are using "optical resin" (CR-39) or optical glass for filters. They are expensive, but Lee is no longer the only good source of glass filters.)
2. Color cast:
   • Stacking lots of resin filters can cause strange color casts
   • With digital, color cast is usually an easy post-process fix
3. Vignetting:
   • Stacking enough resin filters means a pretty thick hunk of junk on the end of the lens
   • At least four resin ND filters (0.3/1stop, 0.6/2, 0.9/3, 1.2/4) are needed to match a single 10-stop filter
   • On wider-angle lenses, you can barely get away with two stacked filters let alone four without vignetting
4. Flaring:
   • Any additional optical device added to the light path adds the potential for flare and ghosting
   • Stacking four ND filters adds a total of 8 additional surfaces which can reflect
   • To my knowledge, no 4x4/100mm filters are multicoated, so their chances of flaring/ghosting are high
5. Versatility:
   • A single 10-stop filter still leaves you some room to also stack on a graduated ND filter to balance contrast

If you want to do some serious ND work, such as 5 minute exposures of sunset along the coast, you should get a 10-stop filter. You'll probably also need at least one GND to normalize scene contrast if you are photographing a sunset/sunrise, which would usually require a setup like the Lee base filter kit with a tandem adapter and an additional independent filter holder (for a total of up to 8 filters)...that is pretty much guaranteed to add vignetting on anything other than a telephoto lens.