I have been living with my girlfriend for 17 years should we get married.

Yes, it will give her rights to benefits if you die, if for no other reason.

Since you are as it appears in a life long relationship. It should not change the relationship put it does give each other some legal benefits at the death of the other spouse.

If for no other reason, to give you both legal rights when it comes to deciding end of life decisions and after.

I don't know what prompts people to live together that long without marriage. I don't think I would do it but if I did, I wouldn't marry just for the sake of legalities. In many places you may already be considered spouses by common law just for the lengthy cohabitation. You can easily stipulate in living wills most everything to and for your partner that a spouse may have, except for Social Security benefits being inherited from a partner who died. So if money from that is not a concern, why do it now?