This is by far the most common handfasting choice.
It's not even that Wiccans are particularly flighty, it's just that some of them have this ceremony, then later roll up at the registry office.
They've had one spiritual big day, with all the fuss around it; then they've become legally married in the eyes of the National Registrar. They just really can't be bothered with pursuing it further.
Then, of course, there are those who quickly learn that a year is quite enough living with that person, thank you very much.
I've heard of the year and a day handfasting very irreverently referred to as 'the trial run'. The couple are one in the eyes of the God and Goddess, but only for the prescribed time. Once that year and a day is over, then so is the handfasting. They are free to run away as fast as they like, or simply remain together without being handfast.
Or, of course, move onto the next stage, either renewing their vows for life, or opting for another year and a day.
Incidentally, there is a mini union incorporated into this one. The physical handfasting comes when one each of the couple's hands is bound to that of their spouse. It remains that way while they leap over a prone broomstick and their marriage has begun.
I've heard tell that in the old days, the newly weds would remain with one hand each tied together for a day and an hour. It forced them to learn how to work in tandem, right at the outset of their union.
However, I've never known a couple who've done it at any handfasting I've officiated at. They've gone for an hour and a minute instead! (Or even a minute and a second.) Cowards.