It only takes one viable cell to grow a culture. In fact, all pure cultures start out as a single cell. The well-isolated colonies on the plates that I posted earlier in this thread are each the offspring of a single cell.
With that said, I look at culturing BRY 96 from a bottle of SNPA as yeast culturing training wheels. It is a relatively easy task to perform because one is usually dealing with a relatively fresh sample, and SNPA is not a high ABV beer.
If SNPA had not existed when I first started to brew, I would have never learned as much as I have about yeast. I am also fairly certain that brewing would have been a passing fad. Bottle culturing led to pure culture isolation, which led to maintaining a bank of yeast cultures on slant. Maintaining a bank of yeast cultures on slant led to a level of understanding of the yeast life cycle that brewing alone would have never touched. Now, I can order yeast from any culture collection in the world without fear that I will not be able to handle the task of growing a tiny amount of expensive yeast into a large amount of yeast. I can also collect and isolate wild stains.
While high quality yeast cultures were more difficult to obtain when I first started to culture yeast, there is no freedom like the freedom to use any culture in the world without having to wait until one of the major yeast manufacturers carries it. It is still worth the effort to learn aseptic and single-cell isolation technique, and it is still worth the effort to maintain a yeast bank. Today, I primarily brew with cultures that are unobtainable via the home brew trade and bottle-conditioned beer, and it all started with culturing the dregs from a bottle of SNPA.
In closing, the number of brewing strains available around the world is easily between an order and two orders of magnitude greater than the number of strains available from Wyeast and White Labs. For example, anyone who loves brewing porter should look into the Taylor Walker strain. One will not be able to order this strain from Wyeast or White Labs because neither yeast company carries it. However the Taylor Walker strain is available from the National Collection of Yeast Cultures as NCYC 240. Conversely, anyone who loves brown ale should look into acquiring S&N's Tyneside strain if brewing Northern Brown Ale or Mann's strain (NCYC 352) if brewing a Southern Brown Ale. All of these strains are available to brewers who take the initiative to learn aseptic transfer technique as well as how to prepare absolutely sterile wort.