I've never understood it. I think for the most part it is naivety, not knowing where they are going, read about poverty levels and automatically relate poverty to televised African drought and such things. Most in the first instance have no idea everything they can bring is available to everyone and usually cheaper than home, so the thought is a lovely one and always appreciated without doubt, no one will ever refuse anything, a something is better than a nothing.
But like you I am stumped why repeat visitors continue to bring stuff, maybe it is a sign of how often they leave the resorts to visit real towns or cities, or maybe they just like giving gifts rather than cold cash, it's puzzling and always has been.
But when asked a direct question of which is preferred then the answer is without any doubt at all, CASH, like Tambo said it is really is that simple.
Even if a member of staff leaves work with a few hundred pesos each day, it is better than all the baseball caps, colouring books and crayons you could ever bring as it feeds them, day to day, it takes care of living costs while on the coast working away so that the entire salary can be taken back to the family home in the city to provide for the kids and needing family there. Specifically housekeeping and gardeners who seem to get forgotten about when money is passed.
A cousin works a hotel bar in Bavaro and pockets about RD40,000 per month on top of RD5,000 salary, he's got the smile the jive talk, the late night last punter pity banter to the overly drunken leaving tomorrow pity forsaken tourist with a few bucks left over down to an art. Barman in a Punta Cana Hotel is a great job