Point Merge can have benefits at some locations. As in a lot of cases it depends on factors like how busy the airport is, how effective ATC already are at achieving high runway utilisation, how much airspace is available etc. The problem is that PM is often seen as a new technique and therefore "sexy" by ATC and Airport managements who are under pressure to innovate and reduce environmental impacts. It is also being pushed hard by equipment manufacturers keen to sell their black boxes, i.e controller support tools, that are needed to implement PM.
The acid test will be real fuel burn numbers as opposed to projections. For me the critical issue for Gatwick with its high intensity single runway ops is, does the use of PM reduce runway utilisation?