As you are probably aware of the fact that the scores of tests like TOEFL and IELTS expires two years after the date of test. The reason they give for this is that one's proficiency in a foreign langauge changes considerably with the passage of time and if someone is not coming in contact with that foreign language then the deterioration of language skills is inevitable.

I do agree with their explanation. But what they say also applies to almost all other subjects. Take mathematics as an example. If someone is not working with math for sometime then his mathematical skills would deteriorate to a significant degree. I remember I once read somewhere that if you want to be good at mathematics, then do it on a regular basis. Even if you are a mathematician but you are not practicing mathematics, then the chances are that when you write 2 square is 4, you would wonder that why is 2 square equal to 4? That is quite right. I have not been doing any mathematics since last year, therefore what I read once exactly explains my condition. What do you on this?
I belive this explanation applies to everything on life .. not only other subjects
if you you were used to running everyday for an hour for example.. whenever you stop your fitness would faint as time passes..
but if you started running again.. it would be much easier for you to restore your fitness than someother guy who didn't practice at all

"practice makes perfect"
I think the TOEFL company is a terrible monopolist when it comes to English-assessment tests. Since their results are (almost) the only ones accepted by most universities in the world, they can:
- set absurd prices for tests (check)
- arbitrarily force people to pay AGAIN after a set amount of time (check)
- not worry about any competition whatsoever, therefore have no incentive to improve their testing practices over the years (check - the iBT was developed just so it could save money).

I've taken the test once and I have to say that expense ranks among the most pointlessly spent money in my life (others were strippers and cigarettes Emotion: smile)
If you have the choice, take the Cambridge certificates, they last for a lifetime.