I hate it. Every time we have a formal party or just a gathering with more than a hand full of people who know each other, I briefly mention what the plan is and give some damn indication of what to wear (e.g. " we often all take the dogs for a walk later, a jacket/flats might be handy"). Basically, if it isn't just friends popping over, there will be some info in some form or shape.
Nobody seems to do that anymore. We are invited to a housewarming, which I'd usually dress down for (everyday me wears pretty dresses with full make-up, apparently that is too much for many). However, they've invited a billion people (up to 2.5h travel time), are renting chairs and have a caterer, all of which screams "semi-formal" or "business casual" to me.
If I'd ask I know the answer will be "whatever you are comfortable in" which is just not helpful. Find a middle line. For example, our wedding was formal attire but I know one guy would be terribly miserable in formal clothes, so I explained that I really don't care what he wears, as long as he could deal with everyone else being in fancy clothes. Give me some damn idea of what to expect!
All we know is, there will be some food and some chairs. I'll make an informed guess in the end but I am so annoyed right now!