I doubt it would be often, and I could get by with just using the plastic corks as I have been doing rather than drop $120. Lots of other things to put my money into. Like more kegs.
Yes and no.
I have used plastic champagne corks with ok success in Belgian bottles. One hurdle is that there is not a uniform mouth size. Some bottles have openings that are way too big and it's obvious you won't get a seal. Some bottles are just slightly too big and you think you get a seal but end up with flat beer. Most bottles, however, do seal ok. It's sort of trial and error. I tend to de-label my bottles so I don't know for sure which brewers use bottles that work with the plastic corks but I seem to think the Chimay and Ommegang bottles are all ok.
The other problem is that the plastic corks don't have as large or tall of whatever the part sticking out above the bottle is called. So as a result, when you put on the cage it isn't a tight fit and the cork will end up pushing up due to carbonation pressure and that will create a sufficient seal (if the bottle has an appropriate mouth size) and the cage will hold the cork in place. The problem is if you push down on the cork at all it will vent the bottles, so you have to be careful that the bottles always remain upright and care is taken to make sure nothing ever presses against the cork. That means you definitely cannot ship the bottles and most likely you will not want to gift the bottles to unsuspecting friends who may not take as good of care with the bottles and then think you can't carbonate beer.
tl;dr - the plastic corks are ok if you're just bottling highly carbonated beer for home purposes and you're willing to risk an occasional bottle of flat beer.