I've always transplanted from pots as well and often move them up from a small pot they have germinated in to a larger one to grow on. I've never had any problems with this method at all.
In a late spring I've held onto plants in pots until they get to the 2/3L pot size. They do grow faster in open soil, but as you say, if it is very wet and the soil is cold and muddy, they would be far better off out of it than in it