Discone antennas see and transmit in all directions.
Yagis focus most energy in one general direction.
Without rotating the RS antenna, you will receive in any and all directions.
While rotating your beam, it is easy to tune out, or even phase out other signals on nearby frequencies.
You will loose the wide bandwidth of the discone. You will loose the omnidirectional coverage of the Discone. You will gain the directionality (is that a word?) of the yagi. You will gain signal strength in the forward direction of the yagi. You will gain reduced signal pickup in all direction except the forward direction.
As far as picking up MARCS, yes you will still be able to pick it up....IF. The yagi will have to be one tuned for 800 MHz and will have to be pointed toward a MARCS tower.
I have 5 bars on scanner, They blink at times, broken transmissions, missed call, garbled transmissions,error rate high.BCD996xt radio is set at 11 and 50 moved the yagi in all directions same happened in all directions.