A: Hmm, let's see. Do you mean use Shikon jewel to increase the power of Tetsusaiga and use his demon form? Shikon no Tama has enormous power and may turn Inuyasha to a full demon if it gets corrupted (hatred and anger adds to its corruption.) In order to stay in his purified form, someone or something which can purify the jewel shard should always be close to him (example: Kagome), which is impossible. Everytime Inuyasha turns into a full demon, he loses a bit of his human heart. If the demon form continues to appear often, he would soon turn into a full demon with no human heart and would obviously kill the innocent humans. Inuyasha did not want that to happen, maybe that's why he hardly used it in Tetsusaiga.