Usually, the MT, the LXX and some DSS say the same. That is great for everyone, this is an example of how purely those pieces of scripture have come to us.
Sometimes, the MT says something, while the LXX and the DSS say another. That is great for us Orthodox, since we rely on the LXX strongly (I'm not aware if any of the miaphysite churches believe themselves to have better versions of the OT); it's really unimportant for Roman Catholics, since the Vulgate is, despite of what was determined in Divino afflante Spiritu,u, still their referential version, considered inspired; and, well, sometimes Prots get mixed feelings about it.
Sometimes, the MT and the LXX say something while some DSS says another. Why bother? Maybe it's something about some shocky subject and conspirationists will go bananas, but it won't change our mlilenial faith.
However, if the MT and some DSS agree on something and the LXX says something different... Well, I don't recall any case like that, but I can't see the Church bother. The Fathers have been sweating the cover of their LXX's and subsequent translations under their armpits for long ago for us to doubt about its precision. We're not a "people of the book", our faith goes beyond the wise words of Holy Scripture. Uncovering the mysterious nominative form of Iounian (Rm 16:7) will not change the fact Saint Paul was complimenting a she-saint.