I more or less zero-carbed for ages (e.g. whatever carbs I got from spinach, the odd bit of cruciferous veg, coconut and balsamic vinegar: easily less than 15g per day). I've been convinced by Optimal Diet reasoning (or more precisely, by Peter) that it's healthiest to eat enough carbs to not be in deep ketosis all the time (~40g for me according to the OD), so as to reduce cortisol and gluconeogenesis, for a while, but in practise had never gotten around to eating that much daily. Eating 40g of carbs on just vegetables is rather trying and whenever I did eat that many carbs I just felt worse (sleepy, slightly bloated etc).
Eventually I got round to matching my practise to my theory however and made a concerted effort to eat up to ~40g via 1 small sweet potato per day. Still, I seem to feel better when I am very low carbing, (with the proviso that I begin to feel worse- slow and sluggish and aching joints and no ability to do intense exercise- after a very extended period of low carbing), so I'm now just intermittently eating about the same amount of carb, which seems to keep me from the long-term slow-down of low carbing while allowing me to not be eating carbs most of the time.
One of my prime motivations for increasing carb intake is because I've recently (last 1.5-2 years) been experiencing problems staying asleep- namely waking up far too early, at approx 4am without having slept enough and without being able to get back to sleep (particularly ironic given my prior sleep problems!). At least one other person I've come across has experienced this and connected it with carb intake and others have suggested that some intake of carb is necessary for optimum serotonin production (incidentally, another motivation for increasing carbs was to increase testosterone) and stop occasional rushes of cortisol, so whenever I do have a serving of carbs it's in the evening, shortly before bed.
Naturally I also focus my carb-intake around workout days so as to benefit from relative muscle insulin sensitivity and aid recovery.