When I was teaching, we were strongly encouraged to join and attend PTA events. Most teachers attended 2-3 a year and there were always several teachers at each meeting.
At my son's school, we rarely seem to have teachers present. The principal is there at nearly every one, and the parent-teacher liaison is there, otherwise they only attend when they are presenting (sometimes in an effort to get money/equipment, sometimes to inform parents on procedural/programs).
Most of our PTA meetings revolve around fundraising and giving to our teachers. We meet monthly. However, if I have a question, I know I can write the PTA and they will get it answered at the next meeting (one of the presentations from a teacher was in answer to my specific question and to receive equipment that cost $800).
I personally don't have a problem with them only attending when they need to as I know from being a teacher, we are asked to go above and beyond often, sometimes at the expense of our own children/needs. My husband's school actually schedules PTA meetings during the school day so that no teachers can attend and so that parents do not have to make childcare and other plans around PTA. This leaves out a lot of parental involvement, but meets the needs of their school. If you have a concern, address it with the school.