Unhealthy Food: If you just that started that diet, purchasing your favorite brand of cookies or crackers in bulk could be a costly mistake. Having a large quantity of an unhealthy product around may be too much of a temptation. If losing weight is a priority, try and stick with the staples and healthier foods in your food bulk buying.
Variety: Often, when you buy something in bulk, there is limited variety -- you use the same exact thing over and over. While this is fine for utility items, for other items (especially many food items) variety is something that's quite important -- and it's difficult to get variety in bulk.
The whole point is to balance your own shopping needs. If you find yourself constantly buying the same products every week, consider purchasing them in bulk. In time, you will find that that buying in bulk can actually be a convenient way to shop and save.