Your starter should be fine for 10 to 14 days if kept cool after it runs its course. Seal it well, put it in fridge, and the yeast will settle making it easier to dump the spent wort before pitching. Erlenmeyer flasks work great for this. (There is equal or greater contamination risk in under-pitching the entire wort and giving some bacteria the time to dominate and overwhelm the yeast.)
White vials and Wyeast large packs each have about 100 billion cells, which is largely accepted to be the minimum required for five gallons of 1.050 ale wort, assuming a high viability rate in the package. If you ferment much more volume than that, or have a 1.060 or greater wort, then you will benefit from having more yeast. One clear benefit of a starter is that you know the yeast were viable from the HBS. (Wyeast packs should expand after smacking, but half the yeast could be damaged and still bloat the package.)
Dry yeast packages of 10 or 11 grams have about 200 billion cells, so properly hydrated dry yeasts offer a simple method to double the yeast count compared to liquid yeasts. And they're cheaper, but you have far fewer options on the yeast strains available.
Yeast generate their esters during the growth phase, so the more yeast growth, the more esters. Growth and yeast quantity is very dependent on the oxygen available for yeast to bud and split before starting to eat. So whether, and how, the wort is aerated is a critical factor.
So the beer style and the level of esters desired, wort aeration, wort gravity, wort volume, yeast choice, and yeast format should all influence the yeast propogation method used for each batch. One size may not fit all. For ex, assuming 5 gals of 1.055 wort, you may straight-pitch a hefeweizen b/c you want a big clove and banana nose, and using the minimum amount of yeast and good aeration will allow for lots of yeast duplication and growth. But you may do a starter on 1.055 oatmeal stout if you want a clean, no-ester fermentation. Or use one pack of dry Nottingham.
Manipulating these numerous yeast variables to suit the brew gives the brewer a lot of control over the final result, and it's applicable to extract and all-grain brewers alike.