As an artist you are throwing your work out into the world, and hoping it connects with an audience. The strength of that connection, its resonance, is, I think, the measure of good art. All art is expression; meaning is its interpretation. Good work will always create a resonance in the audience's imagination, a meaning they draw from it and that draws them to it, whether or not it was intentionally placed there by the artist. Different elements resonate for different people: taste, in other words. Sometimes there is no more complex theme to a piece than beauty or eros (which are still complex themes); but the master artists, the ones that still seem fresh even after hundreds of years, resonate on many levels with many people, even if their main themes are simple beauty.
As an artist it may take a while to find your voice, which to me means discovering and exploring the deeper ideas that inform your work. I don't think that there necessarily has to be a move towards heavy themes-- Gil Elvgren did great work painting pinups his entire life; Leyndecker painted magazine covers and advertisements. But there may be something out there that you haven't run across yet that could give your work greater weight. That's where experimentation comes in. Assignments like the one you described are (ideally) designed to make you step out of your comfort zone and explore. As an artist who lives firmly within his comfort zone, I can attest to the usefulness of such exercises, even while I agree with you that they suck and I hate them.
If you don't feel like you have anything specific to say with your work, that is fine-- practice and experimentation is what we do so that we are ready if and when we do have something more to say. Meanwhile, beauty and eros are themes as old as art itself.