Bees rear brood depending on available resources. If they are low on honey stores, then feeding incites them to rear brood earlier. In the North this has several issues connected to it. First, if it's too cold they won't take the syrup. Second if they start rearing brood too soon they could get caught in a cold snap and "cold starve" from not getting to stores. Third, does it really help any more than having stores does? Or does giving pollen stimulate them more?http://www.bushfarms.com/beesfeeding.htm#stimulativefeeding
Many of the greats of beekeeping have decided this is not productive:
"The reader will by now have drawn the conclusion that stimulative feeding, apart from getting the foundations drawn out in the brood chamber, plays no part in our scheme of bee-keeping. This is in fact so." --Beekeeping at Buckfast Abbey, Brother Adam
"Very many, at the present time, seem to think that brood rearing can be made to forge ahead much faster by feeding the bees a teacupful of thin sweet every day than by any other method; but from many experiments along this line during the past thirty years I can only think this a mistaken idea, based on theory rather than on a practical solution of the matter by taking a certain number of colonies in the same apiary, feeding half of them while the other half are left "rich" in stores, as above, but without feeding and then comparing "notes" regarding each half, thus determining which is the better to go into the honey harvest...results show that the "millions of honey at our house" plan followed by what is to come hereafter, will outstrip any of the heretofore known stimulating plans by far in the race for bees in time for the harvest." --A Year's work in an Out Apiary, G.M. Doolittle.
"Probably the single most important step in management for achieving colony strength, and one most neglected by beekeepers, is to make sure the hives are heavy with stores in the fall, so that they emerge from overwintering already strong early in the spring" --The How-To-Do-It book of Beekeeping, Richard Taylor
"The feeding of bees for stimulating brood-rearing in early spring is now looked upon by many as of doubtful value. Especially is this true in the Northern States, where weeks of warm weather are often followed by 'Freeze up.' The average beekeeper in the average locality will find it more satisfactory to feed liberally in the fall-- enough, at least so that there shall be sufficient stores until harvest. If the hives are well protected, and the bees well supplied with an abundance of sealed stores, natural brood rearing will proceed with sufficient rapidity, early in the spring without any artificial stimulus. The only time that spring feeding is advisable is where there is a a dearth of nectar after the early spring flow and before the coming of the main harvest." --W.Z. Hutchinson, Advanced Bee Culture