[FearNotDaniel]

UK/France isn't such an issue, there's a big stretch of water in between that takes a minimum of an hour to get across by any mode of transport (once you factor in waiting/boarding time).

Not sure how many people make daily border crossings between Spain/Portugal like those I described in my other comment, but even then it is something new if the time difference between states varies with the time of year.

In short, if your country opts out of summer time but your close neighbours don't, all those living in densely populated border areas are still affected by the summer time changes, only with the added complexity that your neighbours in the next suburb don't always keep the same time that you do...