Have been following with interest....have no knowledge/experience of pre-school committees so can't comment there..
Re the books versus playground equipment debate: I have been on 2 very different PTA committees, one for a primary and one for a junior school. Both have been run along the lines that we are raising money for the school. In each case the head has come to us with a "shopping list" and said that they would like us to provide funds for a, b and c, and if possible d, e..etc. Ie they tell us what they would like, and we go away and raise the money.
I would feel very uncomfortable being on a committee where we told the head that we dd not want to buy something that the school wanted (eg books). It is true that we should not really be paying for these necessities, but if in the school's opinion the books are vital, and the school cannot afford to pay them, then why not ask the PTA?
Our head has just retired and said at her retirement party that one thing she would not miss was the budgetting, ie hours spent figuring out what the school could/ could not afford. I take this as an illustration of how much our school, and therefore other schools are struggling now compared to previously. So personally would not want to deny the school anything they wanted - though realise I may have struck it lucky that the schools I have been involved in have made reasonable manageable requests!
Re getting on with other committee members: I think that in both PTA committees I have felt an outsider, especially initially. It does not help that I am quiet and not at all outspoken. I think I may have been lucky in that there have been no/few bullying types on the committees. It has felt cliquey at times, but I try to ignore that.
Re getting a job to do: I have tended to work out what I can do for the committee that is reasonable/comfortable for me, and then offered to do it. Eg on first committee I was ordinary member for a year, and then became secretary in 2nd year. DS2 was at that stage, aged only 11 months. But I felt that I could do my bit by attending 7 or 8 meetings a year, take minutes and circulate them. I made it clear that I could do no more than that. In the committee I am currently on, I said I could send out "begging letters" for prizes for the forthcoming fete. I made it clear that I was willing to do this because it was something I could do from home in the evenings.
Re events being decided on already: I too went to my first ever committee brimming with ideas for events, only to find that they had a year's worth of things in mind already. I felt gutted. But I soon realised that these were the things that were done most years and they worked. So maybe go along to a couple of meetings to see what's planned, and maybe suggest one more event that could be slotted in - though it may end up that you have to mastermind it!
Finally I have been chair of PTA committee for a year, it has been a long slog, and hard work. But rewarding (though I am not sure if I going to carry on). But I would feel really sad if any of the committee members felt that too much was being asked of them - would prefer them to say that they could not do it, and act accordingly.
Sorry this has turned into a long post - but this thread has been very interesting and given me much food for thought.