I noticed Boeing started using winglets in the 90s, like on the redesigned 737s:
(737-800, entered service 1998, according to Wikipedia)
...and the 747-400:
But the 787 eschews them!
...and so does the new 747-8:
And all the press renderings of the upcoming 737 redesign show conventional winglets:
Or what look like a funky version of Airbus's winglets:
What gives? Why do designs from one company use them, then stop using them, then possibly use them again?