Later when I upgraded to the first gen iPhones, I noticed that they didn't have the option to turn off ActiveSync at all. I thought it was odd, but I still didn't mind.
Nowadays, I often leave my phone on DND or Airplane mode for large chunks of the day. I am totally sick of the deluge of mail from what is now 6 separate email addresses. I note that iOS 10 still hasn't got the ability to turn off syncing between certain hours that I know of. On top of that, I have alerts constantly popping up from Twitter, Slack, HelpScout, Intercom et al.
I think the whole "Work can wait" movement is a good one that needs to be front and center at a lot of workplaces (and a lot of apps).
EDIT: Thanks to the responses below, I now realise that iOS 10 DOES indeed have the capability to turn on DND during certain hours only! Winning.