Dynamo Hubs necessary for endurance events?

I don't think its worth having on anything under 100 miles; something like DK200 or trans-iowa, most definitely. Not a necessity but makes life easier. Consider the situation in which you may have to recharge your GPS or being in the middle of B-F-E with no charged lights.

Shimano doesn't make a 12mm dyno hub so you'll have to go SON or SP.

I'm at the point where I'm so used to having light-on-demand that I couldn't see having a bike without dyno lights.  Also the optics on the German dyno headlights still seem to be superior to most battery powered lights. Once you have your bike set up, I don't see the point in taking the dyno off to race. 

Here's a blog regarding "making strong wheels" and has some info on dynamo's...  
https://janheine.wordpress.com/2017/03/14/making-strong-wheels/

As posted previously, if the event is sufficiently short one doesn't need the generator with the associated weight, drag and expense.  There are lots of auxiliary battery options which one could use to recharge GPS, lights, etc...  but batteries are heavy and at some point a generator becomes a more efficient option.

I've got a bike with the Edelux II, a nice bright dynamo power LED light system, but IMHO the beams of my battery powered Light & Motion headlights are much more uniform and brighter.  My preference is a system with two headlights: a narrower spot beam on my helmet and a wider beam on my handlebars.  I also have an older Cygolite LED headlight which has a narrower beam than the L&M and pairs nicely for my uses.

When I'll be out for a long night ride, if I don't need the extra light (ex. going slower or on a well paved and familiar road) I'll use only the dynamo light.  Lots of options to extend battery life. :-)

Cheers,
Greg