How many SC's do you need?
It can be worth it, QC doesn't have a lot of the benefits that Silver has and vise-versa. A combination of QP and silver is better than just silver or just QP in my opinion.
Just to name a few benefits:
*50% Bonus points. (extra points come in handy).
*Preferred seating. (Better than QP and bronze seating).
*Advance seat selection for free rather than $20 per seat per sector. (QP do not get free seat selection).
*Free Q-Tag bag tags. (QP do not get free tags, valued at $49.95 each).
*One lounge pass, good for those times when you've got more people with you than you can guest.
*Priority checkin at QC/Silver counters or Business counters where not available.
*Extra baggage allowance.
*Bags tagged as priority when travelling DOM or Business when travelling INTL. (This doesn't seem to be advertised but has always happened to me.)
There was another thread on this a while ago that might be of interest:
http://www.australianfrequentflyer.com.au/community/open-discussion/need-15-sc-38991.html#post602581