If the Supreme Court reverses the appeals court's ruling and upholds the D.C. gun law, states and localities will be empowered to treat the Second Amendment as the D.C. law does: as a nullity. This will bring the gun control issue -- and millions of gun owners -- back to a roiling boil. That is not in the interest of the Democratic Party, which is supported by most ardent supporters of gun control.The bigger problem of course is the local governments running amok if all laws implied in the constitution are a "nullity". There are some who would like this power very much.
The right to bear arms ostensibly makes life harder for law enforcement because everyone, not just criminals, is armed. This isn't true in practice (see Britain), but that's the argument. Fundamentally the argument for taking away liberties is always the same: a better and safe society. That the facts don't support this, doesn't matter--as long as the government makes an effort to make life better, that's all that counts.
H/T Glenn Reynolds