There is a difference between taxes and mandatory union fees however - taxes are spent to benefit the entire community (theoretically). While mandatory union fees are intended to fulfill the same purpose within the university community, in practice it does not (moreso than taxes), to the extent that students would be better off without them. I don't think anybody here, except the most hardcore Liberal, could argue that this is also true of our taxation system.
Another key difference is the fact that (grossly simplified) taxes are charged according to how much you earn, and how much you spend. The Union/SRC/Sport fee is/was the same for every student, whether or not a student lives on campus and can make full use of those facilities, or a student lives an hour and a half away from campus and spends more time travelling to uni than they do At uni.
Also, consider that the mandatory fee was, until this year, an undeferable fee of up to $600 per student. For many students, this represents easily two weeks earnings. In extreme cases students can choose to eat, or to pay Union fees. I would argue that there are VERY few students who will recoup their Union fees through Union/SRC/Sport activities - and that these VERY few students are those who are in a secure enough financial position to be living on campus anyway.
In regards to the individual issues you raised:
Legal/Financial services: these represent an incredibly miniscule amount of the SRC budget because many of these services are provided pro bono.
Clubs and Societies: Many C&Ses have been forced to close because they are unable to secure Union funding - and have moved to become financially independent from the Union. As a former Society Vice-President, my final duty was to wrap up SECS because the Union owed us over $3000 in funding that was promised to us, but we never received (and ultimately cost our former President $1000, me $400 and our Secretary $400 covering debts - money which we'll probably never see again).
SRC Publications: Honi Soit is a shit rag, but it also comes at a minimal cost to the student body as somebody has to pay for all those advertisements.
Women's Space/Queer Space: Where's the "Men's Space"? Where's the "Chinese Space"? Where's the "Goth's Space"? Women's Space and Queer Space only exist for feminists and "Queer" activists. One of my gay classmates once asked me to fight the SRC on this issue - because he felt the term "Queer" to be derogatory, and that "Queer Activism" aims to be divisive, rather than inclusive in society.
Rights of Minorities: If, by "rights of minorities", you mean terrorist organisations and the politically advantageous ethnic group of the day, then sure. If you mean real discrimination against those of ethnic minorities, then (being of the one ethnic minority that faces greater discrimination in Australian society than any other, with the possible exception of the dirty, dirty Abos) I can promise you that I could find better advocacy from the maggots living in a hippie's hair.
Politics: Why the fuck should I pay for protests and politics, when the student union/SRC can't even provide basic services to my campus? About the only meaningful thing my campus has been able to get out of our Union in the last four years is food poisoning. It's all well and good to say that hippie politics is wonderful. But if it's at the expense of students and the services we are forced to pay for (and it is), then it's not acceptable.