Something not mentioned is while limited compared to a dividing head a spin indexer is cheaper and simpler to use, when you don't need to divide by fractions of degrees. Most of the cheap spin indexers can divide by 1 degree (36 positions + a 1-10 "fine tune" adjustment) increments. No need for plates like on a dividing head. You need a dividing head for making most gears, but a spin indexer can be used (and is faster) for making shapes, regularly spaced grooves etc. Set at 90 degree increments you could mill a square end on a bar, say to make a chuck key, or mill four evenly spaced slots, 60 or at 45 degree increments would mill a hex or octagonal pattern.
Of course even simpler than the spin indexer for these shapes would be square or hexagonal collet blocks.
Spin indexers usually use collets, 5C is most common, but I have one that uses ER32 collets. Dividing heads can usually use a lathe chuck, face plate / tail stock to turn between centers or collets (MT or B&S seem to be common).
So the more complex dividing head is not always "better".