We have far too many laws. Some were appropriate at the time they were adopted, but are now (wholly or partly) obsolete. Some were inappropriate from the start, either bad ideas which arose in response to a short-term issue, or poorly written, or unprincipled. They both clog up the legal system, and can be used unethically through selective enforcement. No-one can be aware of them all.
Society is evolving fast. Almost all laws become obsolete as a result. All? Yes, even murder (think of the ongoing change in attitudes towards mercy killing, suicide, abortion).
Wouldn't this consume legislative time that Parliament now uses to pass new laws? Yes! That's one of its major benefits.
Wouldn't legislators simply renew expiring laws? Human nature strongly suggests not. The temptation to review before renewing will be overwhelming. And many laws will immediately be seen as not be worth keeping when reconsidered.
I fear that the scope of this proposal is so large as to result in its early rejection. However, unlike many other suggestions, it has the potential not only to eliminate current bad law, but to continue doing so in perpertuity. Achieving that result requires such a scope. In other words, it is the systemic solution to the problem Mr Clegg speaks about.
There are precendents elsewhere. For example, the Bank Act in Canada is subject to a decennial review. This has worked well — Canada's banks handled the recent recession better than almost all others.
The 25 year term is suggested because it is one generation. It will give enough time to put the law into perspective. 25 years is also enough time to review all existing legislation.
I believe that only in this way can we both preserve freedom and remove bureaucracy in the long term.