Just as flowers on their own, flowers are pretty useless as money makers, there are always other crops that will outperform them financially, so without beehives, they are basically for cosmetic or gifting purposes.
However, if you do have beehives, you don't need lots of flowers, or even lots of effort, to help them add value to the honey produced. Plant a single flower, and place a sprinkler and a scarecrow next to it, and with no further effort, that single flower can affect several dozen beehives. In that sense, they are definitely worth the effort.
In Fall is where they really pay off - if you have the Artisan profession, a single Fairy Rose will increase the value of every jar of honey within its range from 140g to 952g, almost seven times the value. With a good number of hives, a single flower could deliver an extra 125,000g or more over the course of the season. If you're doing this as a money maker, it's definitely worth planting your flowers using the fastest working Speed-Gro you have available at the time, to get the most benefit from the flower.