If you're anally attentive to managing your data bundle then this is not for you. Fact.
But for the many, many non-anally attentive people out there, who would prefer to spend their time doing stuff other than watching their usage this product was developed.
Using your own case:
You have a 2G bundle. You watch your usage and as soon as you use up your 2GB bundle (or at least once you realise you've used up your bundle), you buy a new 1GB bundle. Now you panic that you don't use it up. So you first panicked that you're going OOB and now you panic that you don't use your new bundle up. A lot of cardiac stress there.
By buying a BB-A 2GB bundle, you can now use it up and just carry on paying the 2GB rate without having to buy a new bundle and having to panic that you must use it. So it does not matter if you use 10M or 10G after your bundle is finished, you pay the 2G price.
So you commit to a rate by buying a bundle at the beginning of the month and now you don't worry to do bundle management for the rest of the month. You don't have to watch your usage like a hawk, neither do you have to watch any subsequent bundles.
But if you're obsessive-compulsive about monitoring usage, for sure stay on the Standard package. You'll be better off.