One of the most common pre-sales questions we get at rsync.net is:"Why should I pay a per gigabyte rate for storage when these other providers are offering unlimited storage for a low flat rate?"
The short answer is: paying a flat rate for unlimited storage, or transfer, pits you against your provider in an antagonistic relationship. This is not the kind of relationship you want to have with someone providing critical functions.
Now for the long answer...
In 1998 I did some consulting for an early, and very successful provider of web hosting. They offered unlimited disk space and traffic for a relatively low monthly price. Of course, the immediate question was how this was feasible - why wouldn't I just pay my $14.99 per month and upload gigabytes upon gigabytes of data, etc.
They chuckled and said (I paraphrase) "the more you upload, the more we throttle you. The little users don't bother us and the big users eventually just go away".
It's hard to say how common this kind of (deplorable) policy is ... but the economics demand that something like this be in place. Bandwidth and disk space are not free, and even the most unsophisticated hosting or storage consumer has the ability to deluge a provider with both.
What this means is that the providers and consumers of "unlimited" services will necessarily have conflicting interests. If your interest is to store as much data per dollar as possible, it will be the interest of your provider for you to store as little data per dollar as possible. The more you utilize, the more antagonistic this relationship will become, as you move from "someone we don't make any money on" to "someone who is killing our business model".
These non-intersecting goals will manifest themselves in any number of ways from bad service and support to "unexplained" poor performance to outright disconnection. As described above, I know firsthand of hosting companies that practice these very measures, and our interaction with prospective customers at rsync.net has exposed us to many more examples.
Here are some specific examples:
The Backup Provider with Unlimited Storage - This is the most common provider we are asked about, obviously. Examples include Mozy and Carbonite. This is the textbook case of what we are talking about here. Traffic throttling, connection resets, and discriminatory support are all examples of things I have personally seen providers like this do to users that don't perfectly fit their business model.
The Web Host with Unlimited Storage - Once in a while a more technically savvy individual will point out that they could get a $4.99/mo web hosting account from 1 & 1 or GoDaddy or someone like that and use duplicity over FTP or something like that to upload unlimited data there. In cases like this, unlimited web hosts have very specific contract language that stipulates that ALL content you upload to the web host MUST be publicly available web content. So the good news is they aren't playing any tricks with the storage they provide (the bandwidth is a different story) but it is unlikely you want to publish all of your data on the web.
Gmail Drives, etc. - Technically very clever, and an example of "things that make me happy", but not worth discussing in terms of critical data backup. Even for personal use, you're moving beyond simply working at cross purposes to your provider, and into the realm of "this tool isn't even designed for this, and can change at any time".
rsync.net, and other per-gigabyte storage providers - We want you to store as much data as you possibly can with us. We will do whatever we can to help you with whatever solution you require to get your data on our systems. The more data you put here, and the faster you do it, the happier we are. It should be noted that we do offer unlimited transfer, or bandwidth, but this is not evidence of a possible conflict of interest. We offer the unlimited transfer as a complement to the data that we want you to transfer here as fast as possible. Since we do not allow anonymous access (or "web" access, or inline linking, etc.) to an rsync.net account, there is no downside for us in doing this.
All of this leads to a general rule that should be applied to all consumption of services: it is imperative that you be aware of the interests of your provider, fully understand your own interests, and make sure that those two things align as well as possible. It is naive to think that any workable solution can be built upon an underlying relationship that is at cross purposes.