I'm up to 2 slicks now. Have a similar bike with 2 knobs that is setup most of the time as a grocery getter, but is also the wife's touring-by-designation bike.

Some of the results from the experiment:
There is a much bigger impact on speed going from knob to slick on back, than from adding the 2nd slick.
big knobby on front or back is noticeably more comfortable for bumbs, and a slight improvement on vibration. A $25 nimus 1.5 with kevlar thicker decorative-hybrid-rain-car-tire thread seems noticeably more comfortable than a $20 no-name 1.4 slick.

I don't really understand the traction arguments on road. The only time I've come close to losing traction was turning too quick too slow, and remedied by pedalling/accelerating out of it. Front wheel traction might come into play trying to go as fast as possible downhill on a twisty path, but that seems more like offroad scenario, and at least not typical city commuting riding style.

Fatter tires seem to handle great on roads, and knobbies appear to have perfectly adequate traction. Handling 1.95 knobbies seems better than 1.4/1.5 slicks, but only marginally.