The first is not so much for daily commuting needs but to have in case of an emergency where a backup egress point would be necessary if the Tram, subway and Roosevelt Island Bridge were all out of service. The likelihood of this trifecta is quite small, although it has happened, but if for some reason the only way off the Island was via the water it would be helpful if more than one location was available for boats to dock.
The second reason involves the planned Tramway rehab and the need for additional transportation resources to cover the added commuter congestion on the subway. If the planned Octagon dock is successful as a commuting point we may actually see commuters traveling North in the morning to get of the Island. If there was a second dock perhaps the water taxi could make two stops with the second one closer to Southtown and the existing transportation hub.
Could the existing pier be modified so that a water taxi could dock there? Was that pier ever used as a true dock? It does not appear to jut far enough out into the channel but the size of the pier would lend itself to passengers boarding and de-boarding with ease.
The concept or a desire for ferry access or a water taxi is not new by any means and the December 1980 New York Times article provided below show that such service was discussed at a level above mere water cooler conversation after a Tram line had fallen causing the Tram to be off line at that time. With the pending Tram rehabilitation scheduled for 2008 or 2009 the issue again bears thought. Granted in 1980 there was no subway service on the Island and I believe the Upside Down Building and its stairway to the 59th Street Bridge might have been gone as well.
Perhaps the Octagon pier (if completed) will pick up the slack from the Tram but with only two stops planned in Manhattan and the only pick up point being up at the Octagon it is still questionable how many residents will take the water taxi as opposed to crowding onto the F Train.
Updated: 10:11 pm
As noted in the comments the “Subway Pier” has been used as a temporary dock for police boats depending on the need. The below photo, courtesy of the Roosevelt Islander, shows once such instance when the police were investigating a floating body that was nearby the Island. See the Roosevelt Islander’s post linked Here for that story.