I dont see why weight gainers cant be used to replace meals. Perhaps you heard that because it is relatively cheaper to eat real food than to chug down a serving of weight gainer(considered both have the equal amounts of nutrient contents)

you're saying you need to eat 5-6 meals a day to what, bulk up? If your calorie intakes are good on those meals, you wont even have to consider weight gainers; provided you also do resistance training to stimulate muscle growth. If not, you'd be just packing on fats