Is shielding necessary on interconnects & speaker cables

Shielding is a means of preventing RF and EMI from disturbing/distorting the signal within a cable's conductor.  This is far more important in an interconnect than a speaker cable due to the relative power of the signal.  RF and EMI can also be prevented from harming the signal in an interconnect by the geometry of the weave.  SO - simple answer is don't bother looking for shielded speaker cables and either use a shielded interconnect or one with a proven track record of good geometry like Kimber Kable.

A lot of cable designers skip the shield in their cables. Teo Audio, Sablon Audio and Acoustic BBQ are at least three brands that fall into this category. They report the SQ as being more lively. 

Noise vs. capacitance.

Most people are fine with unshielded.
The shield in a cable acts as a capacitor plate, so it may introduce some rolloff, which may be desirable.