A melee weapon is for... melee distance. So you might find yourself in the situation where they are too far for your katana, yet too close for your sniper (MED).
I believe the most useful secondary weapon for a silent sniper role is a non-blowback, silenced gas gun. TM Mk-23 socom is probably the most used one, but any NBB (therefore, no noise form the mechanism) gas gun with a suppressor filled with some sort of foam (to lower the noise from the gas itself) would do.
Bear in mind that if you manage to get at katana range then you are probably also in a small knife / bayonet range, so those might be a more practical choice. Yet I have to admit, katanas have a great cool looking factor.
I believe the most useful secondary weapon for a silent sniper role is a non-blowback, silenced gas gun. TM Mk-23 socom is probably the most used one, but any NBB (therefore, no noise form the mechanism) gas gun with a suppressor filled with some sort of foam (to lower the noise from the gas itself) would do.
Bear in mind that if you manage to get at katana range then you are probably also in a small knife / bayonet range, so those might be a more practical choice. Yet I have to admit, katanas have a great cool looking factor.