Never hurts to diversify!
Credit cards that warehouse points to be transferred to multiple partners offer the most flexibility.
Unfortunately with QFF, it's an all or nothing proposition and cards that can transfer to QFF at call are few and far in between (notably the amex plat charge) all other QFF cards are direct sweep, so you don't have a choice of when and where your points go.
Assuming you live in Australia, QFF will always be handy so if there is a lucrative sign up offer, go for it.
The way I see it, there is no harm in building balances in more than one frequent flyer program. The key is to not spread yourself too thin.
If you try to focus on only one program, you can miss out on larger sign up bonuses or better points earning opportunities elsewhere.
Now with two different airline or rewards accounts, it might take you twice as long to build up enough points in one of your accounts to do anything useful, but with two different accounts going, you can alternate between redeeming awards in each and end up with better value in the long run.
So I say go for it.
Personally I earn in both QFF and Velocity as opportunities come up. More flexibility.
i find velocity more flexible and more reasonable in terms of redemption. so while it may look like there are heaps of large sign-up bonuses for qantas, the currency (points) feel cheaper to me. they are worth less.
keith has a table comparing redemption points for different programs: i dont’ think there is any where qantas is the best value (is there?).
In terms of price, I always find Qantas to offer the best value as they have frequent specials.
For points earning, I'm also a loyal QFF as I tend to fly with them or their oneworld alliance partners when I do go overseas. However, I do opt in to Velocity offers when the opportunity presents itself that is too hard to resist (such as the sign up bonuses on different credit cards). The Platinum Edge card I recall offered a free domestic flight every year, which more than covers the annual cost of the card.
Earning points on different programs allows you to be more flexible when flying, but I recommend not to use more than 2 programs at a time for effectiveness sake. Just remember to keep both active so you don't lose your points!
I agree with John. I have membership with both QFF and Velocity, so it allows me the flexibility to choose which airline to fly with (domestically) at the time, depending on fare prices and availabilities. That said, I find the QFF program more intuitive to manage online, then the Velocity program.
If you are flying Virgin domestically and its partners internationally, you should not be swayed by QFF credit card sign-up bonuses unless you have a specific redemption in mind that you cannot achieve on Virgin; for example you want a oneworld round-the-world award.
For most destinations, Virgin redemption rates are lower than those on Qantas, and I suspect they will be for as long as Virgin is aiming to take market share from Qantas.
As John says, Qantas credit cards are an all or nothing proposition, so are harder to justify unless you want concentrate more on QFF. Best options are Amex and citibank rewards, which allow you to redeem on Virgin as well as other airlines (Cathay, Singapore etc). Therefore you are able to 'diversify' while still keeping Virgin as your main programme.
I find the following system works quite well for me:
I am a QF frequent flyer and try to maximise my flights with Oneworld airlines for the status benefits. However, with the Amex Pt Charge card, I get free entry to Virgin Australia lounges for myself and one guest, so there's no particular reason to accrue status using Velocity points (considering domestic routes, where I almost never bother upgrading). So when I have to fly Virgin for whatever reason (cost/company policy), I have my points accrue as KrisFlyer miles in my Singapore Airlines account, which gives me access to Star Alliance airlines for redemptions as well. This way I can build up both Oneworld (via QF) and Star Alliance (via SG) status without sacrificing lounge access domestically.