Yes, quite true about Beethoven. In fact the useage of motival rather than thematic development in the 5th symphony reminds me of Herrmann's deliberate use of truncated motifs in most of his film scores and - signifcantly - his later chamber music such as "Echoes," a brilliant string quartet. Beethoven was the first to significantly use a deliberately, consciously simplified motif to show just how much he could do with it. And this was even though he was quite capable of creating much more elaborate melodies.
However, to return to film scoring themes, it is just as practical to create an entire score out of a single melody (with a few interludes). This can be observed in the great score to Somewhere in Time by John Barry. He simply didn't need much more after he composed that theme. Whereas a lesser composer will have to bust his ass doing scene after scene separately, Barry could kick back and do a slight variaton here, a few chords there, once he had that perfect melody played by rapturous strings. So that is the OPPOSITE situation from clever motif development and variation.