It's not worth to collect for future monetary gain. DVDs, like most things factory made, only become rare if there was a recall or end up going out of print (low original prints add to this factor); though if it's out of print that doesn't mean it won't one day be in print again, it's digital afterall.
It's also not worth collecting if quality of product is a factor for you. Again, it's digital and you can get a "copy" of the actual film at the same standard. Furthermore, there will always be a new improved technology that will make the standard obsolete. If you were buying 35mm prints that would be a different issue because it's analog; there is wear and tear on other prints and you can bank on future prints not being produced.
It is worth collecting if you see your collection as an expression of your tastes. Especially if you like to put your taste on display; some form of stamp on your personality. In that case, you getting down to the essentials is, in my opinion, a much more concise expression of what you dig. Even though it gets borderline pretentious to collect with the intent to show your stuff off, it still is expression.
Most importantly, it's worth collecting to support the artists you like. In an age of piracy, it's important to support the people you want to see the value of continuing to do what they do. Even though it's probably inappropriate for me to say, the little guys need your help. I'm at a point where I'd much rather DL something that's mediocre hollywood fare to directly effect the industry; by not supporting it with my wallet. I know it's wrong because i'm still watching it, i guess i just don't care.