You always have been able to opt out of AFPS but why would anyone? If you are someone who knows you are going to leave the military and have a successful most mil career, then lets assume you will want to be able to max out your lifetime pension allowance. You may of course, be in a rank that is going to cause you to breach the allowance anyway, or you may anticipate it.
But as an aside, in principle, you now have far greater choice in how you receive retirement income - would you want your pensionable allowance to be tied up in a public sector final salary scheme which might restrict delivery of income, and offer not so good dependents benefits or would you want your pension in a scheme which offers access in a different way and more diverse estate planning options.
There are lots of downsides of course, overwelmingly so; the funding would have to be contributory and you'd lose death in service benefits for instance, and for the vast majority of people you wouldn't even consider it. It might also be that a divorce affects your accrued benefits, with you as either a pension credit or debit member, and that is something else to be considered.
As a further aside, the Financial Ombudsman has also recently ruled on QROPS which seems to make them more core to more people's planning.