Uhh, I'm talking about desktop Ubuntu, not server installs where you don't have pulse, xorg, a DE and a shedload of other software. Getting a server upgrade right shouldn't be too hard, but when you add on another thousand packages or so, it makes the upgrade process much more likely to have breakage.

I've had relatively smooth upgrades on 14.04+. Anything 12 and before has always been a bit tricky. 14 -> 16 I'd rather just reinstall since it leaves all the old upstart stuff laying around and can be confusing to troubleshoot.

Prevents PHP upgrade issues by not forcing that alongside the distro upgrade. Have php5 & php7 packages

True, but it needed to happen (and I think apache 2.4 was forced).

Generally it takes us about 12-18 months to get round to moving things on, and an OS upgrade is a great opportunity to kick us into a 'do it now' state.