Sugar water is not a good source of nutrition for bees. This practice has been researched to have contributed to reduced immune system response which makes the bees more susceptible to pathogens. Colony collapse disorder is one link to this practice. Flower nectar has essential minerals and amino acids that bees have evolved to digest. Many chemical characteristics from nectar (manuka honey for example) remain in the honey and there is a great deal of chemical diversity in honey. However, there is a ratio of honey a bee colony can sacrifice and sugar water supplements calorie requirements. I would talk to an experienced bee keeper (preferably sustainable) on how they determine how much honey can be harvested without jeopardizing the colony health. Bee keeping is like medicine, its called a practice for a reason and thats because you are always learning something new so you will need to expand your training beyond one class designed to get your feet wet.
Sugar water is like fast food, but right now it isn't easy to find anything healthier to give them, unless you have leftover honey from * YOUR OWN* hives from the year before. It isn't that sugar is a good food, for most of us it is the only food available for the bees when they need extra food. Great? no. Are there any alternatives? Very often not.
Beekeepers check their hives during the first warm weather, partly to check the stores of food. If the bees do not have enough food, then the hive must be fed or the bees will starve and die.
Honey is a healthier food for bees than sugar water is, but honey from another bee yard might contain disease spores. Many beekeepers will therefor feed honey from their OWN hives, but no other honey at all. A small bee keeper, like myself, might not have any excess honey to feed in the spring.
In a nutshell: honey and pollen are better for bees, but unless the beekeeper produced it it might carry disease. If a hive has insufficient stores in the spring, you must feed them or the hive will first produce few young, and if they run out of food when it is still is too early for flowers the hive will starve. Sugar is not that great a food, it is fast food for bees, but it is often the best food that we have.
Most bee keepers check their hives as early as possible. If there is enough food good. If there isn't the bee keepers feeds sugar water because otherwise the hive is at risk. And, if the hive is light on food in the fall the bee keeper feeds. For that matter, when buying bees the bee keeper will feed, to give them nourishment until the bees find out where the flowers are.
Sugar water is not nearly as healthy as honey for them to eat, but it beats letting them go hungry. It is the equivalent of eating a burger and fries for lunch. Fast food isn't healthy either, but most of us will eat it rather than not eat at all.