Many people dont mind using these kind of FB's because it gives them an extra edge going forward. The best FB's in the world are good at both ends of the pitch. Daniel Alves is ofcourse a prime example. Also think Roberto Carlos, Ashley Cole etc. We're beginning to see it now in Bakary Sagna at Arsenal as well.
I usually never go for these guys (Ratinho, Marcelo) as FB's. Had Marcelo is one game and I always played him LAM. My preference is someone who can defend well and also cross if needed.
The attacking FB's like Marcelo, Ratinho work best if you go with 5 man defence. You can have 3 CB's who stay back at all times and Marcelo and Ratinho as wing backs bombing down the flanks. It really does work very well especially if you play counter-attacking football. That's when these FB's are at their best. Not a good idea to go for them in a 4 man defence because if you come up against teams with good wingers (Man Utd, Valencia etc), you'll get screwed. And another reason why people go for the attacking FB's is because they like to play that way. Cant really fault them for that!