either 5.03 is beta (some users already have it, not everyone), it happens when they add a lot of new features.
either sony didn't think it's mandatory because it doesn't fix a vulnerability and/or doesn't add any online/interacting with other users feature.
Sony is not so picky anymore for no online access when not on the latest firmware. It is however stated in the PSN+ agreement that you can not ask for a refund of any sort when not on the latest firmware.
It depends on whether the update does include changes regarding the PSN API in general (and of course if it's a vulnerability patch).
If not then you won't be force to update when you try to get online. It would be a PITA if you have to back support 65535 different iterations of the old API.
(But they did it mainly for the sake of vulnerability patch I think.)