It is also possible to hand hold the filter on the end of the lens. It is far quicker than using a holder. Do this a lot myself. Provided you don't press the filter on the lens when adjusting position there is no risk of scratching. Not convenient for long exposures though.
A hair elastic will also work to hold filters in place. Just slide it on and it will catch on the edge behind the hood mount on most lenses.
Holders really come into their own for stacking filters as they let you easily adjust each one independently.