One critical point that seems to have been omitted in all the responses so far relates to the definition of durability. Specifically, a wood species' durability classification is derived from degradation of the wood over time in ground contact. So, a non-durable classification means a lifespan of less than five years when the wood is, for example, buried in soil as a post. The post will rot at the base and in the ground long before the part only exposed to the weather, which would last probably two or three times longer.
Having highlighted that point (I'm nothing if not a bit of a pedant sometimes, I'm afraid), ash still wouldn't be high on my list as a candidate for bench seat and back slats, although it does traditionally have uses for external artefacts, e.g., spades and forks, sledgehammers and, as already mentioned, Morris woodies, etc. Ash simply doesn't age well when exposed to weather, and it wouldn't last anywhere near as long as a more durable material such as European oak, especially if, like me, when I make wooden structures for outdoors I prefer to apply either no finish at all, or quickly apply a coat or two of linseed oil just to provide a nice colour when the item is new. The linseed oil gives little or no protection really, but it does prettify the item for maybe a year, after which the bacteria, fungi, bugs, etc, just get on and do their thing.
The primary reason for this little or no finish preference of mine is simply down to the fact that 99% of clients will never keep up a regular maintenance routine on something like a varnish finish, even if they swear blind they will, and peeling and flaking varnish is a cause of accelerated wood degradation as compared to never varnished. All of which, incidentally, can lead to customers complaining a couple of years down the line about the quality of the finish, the black marks in and on the wood and its general shabbiness, having completely forgotten their need to maintain the finish on a regular basis, ha, ha. Slainte.
