As always fred these things are a personal choice.
We just got fed up with difficult taxi drivers, vehicles in poor condition with poor air-con, grubby and dirty with ripped upholstery and the constant negotiations on price. So many times we jumped out due to no air-con or the refusal to put on the meter.
Yes Grab (and Uber) can be marginally more expensive by a few Peso's but for me it's a no brainer.
In Manila it's so much nicer and more relaxing to be a 'registered' passenger' inside a 'registered' vehicle.
Depending on the needs for transportation, also check out LRT and MRT routes ;-)