First Statement of a Theme
When composing a piece, present your main musical idea for the first time in the clearest direct and most unobscured way to your audience so it is actually being perceived as important and remembered for further appearances where it might be transformed, accompanied etc.
By that you make sure that your audience understands and memorizes your key idea. The urge to “embellish” a theme with secondary lines etc. is usually based on the fact that you have been working and hearing that theme so many times that you want to freshen it up a little with additional material.
However remember that you audience never heard that theme before and to them it is a completely new and fresh idea to process. So on the first occurance there is no need to add secondary lines etc.. Save these things for reoccurances of the idea later on in your piece.
If you listen to all the successful main theme pieces from the last decades, you will find that practically all of them follow that rule when they present the theme the first time.