I don't think I'd state it quite that way; but hey, your thread, your phraseology.
Locals are allowed into any resort at which they can afford to vacation, whenever they want to pay their way. Having said they tend to want to vacation when they have an extended weekend which will permit their maximum relaxation (in their own way) much the same as any group anywhere else in the world.
The best thing to do if you are looking to avoid a crowded resort (of any nationality) is to avoid particularly busy holidays such as Easter, Christmas and New Years. Also try to avoid long weekend holidays like US Thanksgiving and Memorial Day as well as the host of Dominican Holiday Mondays (depending on the year they can be quite plentiful) like those which might be found here:
DR1 - Travel - Holidays 2009
I have vacationed when there were and were not Dominican long weekends and while there can be a noticeable influx of guests it never became so crowded that I felt vacation deprived. If you cannot avoid a long weekend the following strategy will minimize the impact of the extra guests:
1) Schedule an excursion for one or more days on that weekend vs. midweek.
2) Spend a day at the resort beach as at resorts most Dominicans tend to use the pool, they have beach access the rest of the year, if they choose to use it.
3) Schedule a dinner at an a la carte as Dominicans tend to use the buffet.
Resorts, like any other business, will try to maximize income but will not intentionally oversell to the point of negatively affecting their client's vacation experience. They might get it a bit wrong on occasion but generally they hit a reasonable balance, IMHO.
Have a great vacation, wherever and whenever you choose.
Gregg
