This is a good move by SIA to charge the economy class passengers for occupying the seats with more leg-room. This move makes it easier for the cabin crew who has to contend with some passengers who would demand such seats inflight, even when they are already occupied.
But the above means those who are willing to pay more (US$50 ex SIN) may be those who are not abled bodied or understand English. These seats are usually situated next to the exits and in the case of an emergency are they able to assist the crew to evacuate other passengers? These seats are supposed to be occupied by passengers whom the crew deem fit to assist in the evacuating of fellow passengers should the need arises. But what the heck,bottom line comes first right?