Except this wasn't true in Legion at least not for the class I played, Balance druid. For that spec, haste and mastery are roughly the same value, with versatility and grit being noticeable less valuable. So, at any one time I could look at my gear and say "Man, mastery is fine but I'm not getting any haste gear" and reforge some versatility to haste. When I have enough, I could reforge to mastery. Or I could do the other thing and if I was getting tons of haste gear bit not a lot of mastery...
it's basically a protection against getting a ton of drops with crappy stats on them. It's disheartening to get an item level drop that is actually not an upgrade because much of the stat budget went into stats that aren't good for the spec you're playing.
Where reforging fails is when one secondary stat is much better than any of the others.