This idea is very important to spread the message in society that same sex relationships are as valid, meaningful and human as heterosexual relationships and that any couple wishing to make a legal commitment to each other should be equal within the law.
Whilst the establishing of civil partnerships for same sex couples could be seen as progress in the law and society, it could be viewed as something of a condescending concession, i.e. 'You can have some sort of ceremony, but not marriage because you are not a 'normal' couple.'
Similarly, heterosexual couples who wish to make a commitment to each other, but do not wish call each other 'husband' and 'wife' should be allowed to be known as legal 'partners'.