I'm really not sure about this subject.
Of course, national stadiums are used for cup finals, among other things, as well as for international games and it means the national side have their own stadium.
However, I don't think it's that important. If the national side played in different stadiums across the country, it would be fairer and would mean that more supporters would get the chance of seeing their team.
Of course, a country like Scotland needs a national stadium as generally the other stadiums available aren't to that high a standard for international football. But for countries like England for example, their stadiums are to a high standard.
There are some countries, like Italy and Germany I believe for example, who don't have a national stadium and it works fine.