I have had the same question in the past, whether an unusual hemline will make something look dated. But as you point out, that very feature can add a little verve to something that would otherwise be very classic and maybe feel a bit "boring."
A month ago, I bought a trench from Anthro that has a handkerchief hem. I hesitated because of the hem, but I'm very glad I got it. The hem does indeed give the coat more playfulness and movement than if it had a straight hem. I'm hoping that since the rest of the coat is a very classic cut, it will stand the test of time.
I also bought a skirt last year with a handkerchief hem from WHBM. Again, it adds a little edge to what would otherwise be a very classic dress. The dress is a floral print, and it would be a little too feminine for my style without the special hem (I think the hem adds in some angularity to constrast with the softer fabric).
So my solution is to buy just a handful of pieces with unusual hems. That way, I can wear them frequently while the trend is still fresh. If I get too many, I run the risk of not being able to enjoy them sufficiently before the trend changes.
