Like said before, the only reason you'd want it is if one of your Pokemon have sunny Day. My strategy is to have a Fire type with fLamethrower and Sunny Day and then swithch into the SolarBeam Pok3emon, bur since your using Sigilyph, SolarBeam is not STAB (Not forgetting its a two-turn move)...
However, it Covers its Rock weakness, but Id prefer energy ball for that.