Your points apply not just to the Guardian, of course, but in a sense all print newspapers.
I agree with the serendipity issue. The inclusion of stuff you're not normally interested in may be annoying but also helps discover things you might not otherwise have.
There's another point also - if people are "too" selective about what they read, they may find themselves only reading viewpoints which they agree with and be confirmed in their narrowness, rather than those which challenge them and make them think. Twitter may provide serendipity but as one mainly tends to follow people with similar attitudes, it won't necessarily provide the alternative viewpoints.