Good points in there
I interpreted (it's the internet remember) it in a way of "Brian uses them so they must be fine for me" kind of idea. Which is why I noted out his collection is vastly different (obviously a building dedicated to the "hobby" and also the types he is keeping (big things like a retic). If you have a few adult snakes, thats a very different situation and as such think they should be in an "enclosure" (something more advanced than moulded PVC).
As a development from your point, the "interest" and "reward" in the hobby for you is the up and coming differences with your MS, why hide these "new and interesting" products away in a tub. Years and years of line breeding stashed in a tub doesn't make sense?
There isn't years and years of breeding yet. Maybe a few generations. They aren't really stashed away. When you have dedicated reptile rooms to house your animals, they are generally out of sight anyway. Whether they are in a tub or in a viv, when I go into the room, everything gets a look at. Sure many in the vivs are hiding and many in the tubs are hiding. I see both out and about and acting the same as each other all the time.
I guess because space and time becomes an issue. If you believe that the appropriate sized tub will house the specimen well then it means you can keep more of the ones you have bred and want to re-breed and you have a larger project base to work with. It means cleaning is quicker and easier and inspecting your collecting is a lot quicker. Don't get me wrong, like I said I use enclosures mostly. But tubs are very good for larger collections.
Sure the intricacies of larger collections may require the use of tubs to a degree, but over the lifetime of the snake, surely there is a better option.
If you think about it from a snakes perspective rather than what a human needs or what a mammal needs tubs aren't really a worse option. You have to select the right sized tubs and set it up appropriately for the type of specimen needed to be housed. The requirements are no less important in a tub as they are in an enclosure.
You're only holding back for a year or 2 to strengthen the bloodline then moving them on? I guess in a "rotation" there will always be a new lot of holdbacks which means the tubs will always be filled?
No its not holding back just a year or two. To try and breed similar traits and patterns we are talking about keeping multi generations for a long long time. There will be some rotation, but you also have to "prove out" genetic traits and traits that are not passed on at all. So you need to keep a few generations to breed back against etc.
So as well as the older hold backs you are adding new ones each year. This is where utilising space efficiently comes into play with people who enjoy breeding with a goal in mind.
But yeah....seems like a waste to have an "impressive" snake (subjective), stashed in a tub. May as well have a rubber snake too
Generally you don't just have one impressive snake. You'll have a heap of them. I don't think anyone who uses racks don't have a few display vivs scattered around the house for the joy of observation. I'm sure some people rotate animals in their displays too. So you might have 10 nice snakes out on display and another 30 - 100000 in tubs. I think you might be thinking from the perspective of a smaller amount of animals. And to be honest I don't know anyone with just a few animals who use tubs. Tubs seem to come into play when you find yourself spending too much time making 20 enclosures look pretty, too much time washing poop and glass on enclosures, and you want to best utilise the space you have available, whilst still providing your animals with the essentials that they need.
*also a rubber snake cant be used to produce the desired progeny you are keeping all the snakes for. Again it comes down to enjoying the process of line breeding as opposed to someone with only a few snakes who doesn't breed but just enjoys having a cool pet to look at and play with. For me I think the best of both worlds is the way to go 
