Do's: there's a contraction - the 'e' is missing

Think about it:

VERB: watch - 3rd person singular "he watches"
NOUN: a watch - plural "watches"

The plural of a noun generally follows the same spelling rules as the third person singular of the verb in the present simple.

But with "do" that creates a pronunciation problem, so you need the apostrophe as the plural of the noun is a contraction.

Not to mention that fact that it helps avoid potentially embarrassing misunderstandings... I mean, the "infamous Honey Dos!" - is that a Bond girl or a porn star, because it looks like a name to me!!!