IANAL, but I think the most important aspect is to give you cover for the kinds of self-protective measures you might need to take -- like disabling/blocking certain users or IPs, deleting certain user content, etc.

In the absence of any TOS, a litigious disgruntled user might portray your actions as unfair according to some implied standards of continued service, etc. By making your scope of actions explicit before the fact, their argument becomes weaker.

That is: you're unlikely to be able to hold a TOS violation against a user for any damages, but it will provide cover for your own self-defense actions.

The TOS isn't meant to protect the users. It's the user's job to protect the users. The TOS is to protect the site.

Say someone uses your site which provides a free service. For whatever reason, the service is disabled for a day. Now let's say a user decides that they were harmed by the service being down and decide to sue. It's a good thing you have a TOS that states "The services offered through this site are provided as-is and without warranty. We may discontinue service or cancel accounts at any time with or without reason."

I understand that there is a potential for fraud from the site, such as when the site decides to add a line after being sued that retroactively protects them, but the burden of proof is on the accuser and there are ways of checking revisions to a site (checking Google's cache or recent edit logs could help to find the truth).

There is a potential for fraud from the user as well - taking your example:

The user can edit the html, print if off and say that /that/ was the TOS he agreed to.

I don't know if Google cache or the wayback archive are usable as evidence, that would be for a court to decide, but there is always the possibility that they didn't catch it, and the user can always say 'on X date, when I joined up, it said this and this (with the fake printout)', now the site has to prove otherwise.

In fact the user can just claim he never agreed to such a TOS, that no 'I have read and accepted the TOS' checkbox was displayed to him at any time.

That's why there is another clause that states "We reserve the right to modify these terms and conditions at any time and for any reason, with or without explicit notification to you."

This is why you always hear "be sure to get a good lawyer" when discussing legal issues. It's their job to protect you from liabilities. Sure there will always be cases where someone could present false evidence, but this is why we have a court system.

I think this is really frightening. Can this really happen? Does anyone know the subject well (a lawyer maybe) and can give a definite response?

Just because there isn't a paper contract doesn't mean it isn't binding. Make sure a lawyer looks at your terms page in addition to writing the draft. I know that I am indemnified completely with my TOS.

Also, IMO just the presence of a clear TOS is a deterrent for all but the dedicated abusers who don't care about prosecution.

Are you sure you are completely indemnified? There are several jurisdictions where liability from negligence cannot be contracted away. The same is true for breach of warranty and strict liability claims.

Another example is if you are a web service that doesn't comply with DMCA provisions, you may be liable for your users actions.

What I am trying to say is that a TOS does not indemnify as a matter of law. The question of fact for each specific issue will be sent to the jury who will decide your fate.

Whether a TOS creates a contract could also be decided by the jury. They aren't contracts 100% of the time.

paper contracts don't mean anything either. Even if you have one and win the case, the other party can just not pay

I disagree. Some people may be considered judgment-proof, meaning they have no assets to be sued for (O.J. Simpson, for example). However if you win a judgment against someone who has assets but refuses to pay, a court could order wage garnishments and even asset seizure. Basically the courts can make you pay by taking from you. (At least, this is how it works in the USA.)

Of course cases with citizens of different nationality are different and much more difficult to work with, even with a paper contract. Both governments must be willing to hold their citizen up to the agreement in the contract. If I have a contract (printed, signed, and notarized) between myself and the deposed king of Nigeria, it would be hard for me to take the ex-king to court.

I have one for my service in case I need to remove content or close an account for some legal reason (copyright infringement, illegal activity, etc). As far as proving someone agreed to it, you can't sign up for my service unless you check the box saying you agree which should be proof enough. I am not a lawyer though.

Protect yourself.... by registering to and continuing to use your site users agree to x,y,z.

Establish trust... if your terms are reasonable then they show you are serious. The lack of a privacy statement and terms looks suspicious to me.