Firstly, remember that the play clock is governed by what stopped the game clock. If the ONLY reason for the game clock to stop is the injury or helmet off, then the play clock is governed by 3-3-9-b.
The primary reason for 40 seconds for a Team B injury/helmet off is to reduce the benefit to Team B for faking an injury. If the play clock were to be set to 25 after a Team B injury or helmet off, Team A would not get the benefit of the full 40 seconds they would normally have to make personnel changes, make a play call, etc., or simply let an additional 15 seconds run off the game clock, all as they would be able to do between downs in any other situation. If the interruption is no fault of their own, and in the absence of any other reason to interrupt the play clock, they should get the full 40-seconds to put the ball in play.
Otherwise the play clock is set to 25, as it would be for any other administrative stoppage. 25 seconds reduces the benefit to Team A for faking an injury.
In the event of a helmet off, in particular, game officials should get that player off the field immediately, and restart the clocks ASAP. Neither team should get the benefit of additional time to rest, make personnel changes, etc. For an injury, we simply have to allow the "injured" player to get off, or be removed from, the field at whatever pace is required.
