Wet samples are definitely the best way to match paints when you are trying to mix a paint to match another color.  It allows you to add additional paint to the mix without having to wait for things to dry again, however it has a number of problems.

Not all paints use the same base, and those different bases will change the final color when dry.  Most are familiar with paints being a bit darker when wet, but that is not always the case.  Many paints use vinyl additives (they help keep the paint a little flexible when dry and less prone to chipping).  Depending on how much, the paint will actually darken when it dries.

Consider adding a bit of color to normal PVA glue.  It will result in a pastel looking color, however when it dries it is clear and the color shows more or less like it would if it were just the paint.  If you were to take a paint like Vallejo Model Color, which contains little other than clear acrylic for the binder and try to wet match it to a high vinyl based paint (Gamesworkshop's Foundation line is one of the most popular now - though the old Partha Paints also were high vinyl content paints) they would be far off from each other when dry.

Not only that, but it is much easier to have a dry sample mailed from the other side of the world.