Agreed. I have no problem with them showing ads and trailers, but they should be legally obligated to provide the actual starting time of the feature film as well -- so if I don't want to see that stuff, I can come 25 minutes "late" or whatever.
It's pretty easy to estimate when the feature will start, so you can skip the ads if you want, but if you wait to go in you're going to end up sitting in either the front row or the back row. So, your choice is either crappy seats or spoilers. Personally, I've started bringing in headphones. Closing my eyes and drowning out the trailers has vastly improved my movie-going experience. It's nice to see a movie without already knowing every major plot point in advance.