Anyway, what happens if you whole hard drive dies?
What if you want to reinstall everything from scratch?
There is just too many what ifs
I thought about this and while I think having a CD is better for these reasons, I don't think it would leave you up a creek without a paddle.
Either you have an OS that supports Mac App store so you'd have a CD that would at least install that OS (and therefore you could install old OS and go back to Mac app store and reinstall Lion) or you'd have to buy the Lion CD anyways (but in this case if you lose the Lion CD you may be w/out Lion).
So, while the app store does have the advantage that if you buy through them, long as you have the CD from the previous OS (and probably not too expensive to buy a CD off of ebay, don't know, haven't checked) you can re install Lion. WHere as if you buy the CD and lose it, you'll have to buy Lion all over again (and I am betting Lion won't be as "cheap" as Snow Leopard as it isn't considered an incremental upgrade).
But... it also means more hassle if your hard drive does crash cause you'll have to install an OS twice.