Hard copy book should be able to be cited because they are credible sources. I don't even consider it to be significantly inconvenient. Chances are there is probably a virtual version available somewhere that other people can look up. If nothing else, you can probably find it at a library somewhere or at least find a summary with comments of it on the internet to decide whether or not it is truly credible. I have cited college textbooks as sources before and I consider that completely credible.
There is no reason to discount literature cited as a source. It is just as valid as any other source and is widely used. More and more literature is becoming accessible online or have some basic information about it online. On the claim that it is unfair to the other person, it actually gives them an advantage. They can simply say they are unable to confirm their sources, and I'm sure if it is a worthwhile argument to rebut, it is not based solely on the source and can be attacked in different ways.
I believe it is unfair for the other person debating to have unaccessible sources that are used to provide arguments against him. Unless people have access to web databases, it is very difficult for them to get their hands on such sources. Moreover, most people that have free access to such web databases are university students, as their universities provide this service for them, and this does not include all DDO users.