There's a difference between non-compete clauses (that prevent you from working for a competitor after you leave - and are no longer being paid by - your now-former employer) and gardening leave (where you're still employed and getting paid, but you're sent home for the duration of your notice period). The idea behind the latter is that, by the time your gardening leave is over, your knowledge of clients, etc. is out of date and/or (in situations where you've been the primary point of contact or managed the the relationship with the client) you've been replaced.
In most EU countries, non-compete clauses require compensation. In Germany of Belgium, comp' must be at least half the gross salary, for the entire extent of the clause (limited to 2 years in Germany, 1 in Belgium)
And the NCC may still be done away with for being "unreasonable" e.g. it can not cover the whole country and prevent the former employee from working in the field.
