One advantage is that it is much easier to create multi-pixel receiver systems. Having two receivers at the focus is like having two telescopes! So for survey s

One advantage is that it is much easier to create multi-pixel receiver systems. Having two receivers at the focus is like having two telescopes! So for survey science, it is much more economical than building 50 telescopes with a total equivalent area and one receiver each than building one telescope and 50 receivers. However the main reasons we often don't build big single dishes is that (1) it's very hard to keep the surfaces from deforming from gravity and heat (the GBT for example needs to be accurate to 0.2mm over two acres of surface), and (2) the bigger the telescope gets the bigger the sail with respect to the wind, which makes it hard to point and keep the pointing accurate.