Why would I want reversible blade spin?
There are 2 good reasons why you would want reversible blade spin; it gives you control of the direction of spray of the mud and debris from the blade, and it enables you to cut horizontally.
Controlling the mud
With the slurry from the blade flying onto yor excavator, it takes very little time for it to obscure vision and get into every nook and cranny.
The problem with the first is obvious - the operator can no longer see what he is doing.
The second increases wear and tear on the excavator by introducing very fine, abrasive grit into the working parts.
The alternative is slurry flying from the blade onto some neighbouring property. The fine grit can be difficult to remove from brickwork, gardens and swimming pools.
Towards the excavator or away from the excavator are your only two options. Having reversible blade spin gives you the choice, regardless of which way you pick up the saw.
To cut horizontally relies on the slew control of the excavator.
Not only is this a fairly insensitive control and difficult to do gently, it also has a lot of backlash.
If you spin the blade in one direction, the blade will push back against the excavator, taking up the slack in the slew bearings. This way you can cut.
If you spin the blade the other way, it will pull into the cut. The loose movement in the bearings allows the saw to be pulled into the cut, and the blade jams.
With single direction saws, you cannot choose the direction of spin which greatly limits your options for horizontal cutting