When I was getting married I thought about this a lot. I ended up changing my last name to my husband's. The deciding factor -- our future children. I really wanted our family to have the same last name, plus it was extremely touching when my husband said that he is willing to take my last name if I wanted, just so I can keep an "indicator" of my heritage. I guess that's when I was certain, if this man is willing to do something like that to make me happy, then I would be happy to have his name. We have decided that we'll give our kids my last name as their middle name, just to give them an extra ingredience.
I have to say that it's also quite an interesting experience, it's weird to be of Armenian origin and have an Irish last name, imagine if my first name was Tsovinar or Zamroukht.
I have to agree with Maral and anoushik. It's really an individual choice, I don't see what the big deal is, one should do what they find most convenient or practical to themselves, rather than adhering to some senseless principle. I am not a fan of two last names, it gets too complicated and too long, and it's true, down the road things will get even more confusing for the younger generations. In terms of identity, it's really not, your last name is your father's, from one man to another, so you just chose which man's name you'd like to carry, and if you love them both it could be a tough choice. Now your first name is your own, that's what people know you by, but that could be changed also. Nothing is really constant, so who cares, just do what's reasonable and what you desire most.