Is the HAVING clause redundant?

The following two queries yield the exact same result:

select country, count(organization) as N
from ismember
group by country
having N > 50;

select * from (
  select country, count(organization) as N
  from ismember
  group by country) x
where N > 50;

Can every HAVING clause be replaced by a sub-query and a WHERE clause like this? Or are there situations where a HAVING clause is absolutely necessary/more powerful/more efficient/whatever?