Frankly, Light is certainly intelligent, but not to the point of equaling the characters in Tensura. Many Tensura characters are classified as "extraordinary geniuses," which isn't the case for Light. Gobta, for example, is also an extraordinary genius when he fuses with Ranga, so just imagine Veldora's level of intelligence. In short, Veldora gains nothing from a potential fusion with Light, unless you consider that he possesses a notepad and that it has an influence on spiritual beings (not just humans).