I guess the question was WHO saw it as necessary. Early settlers wanted the land, pure and simple. The politicians wanted to give to to them, both to appease the people and to avoid the trouble natives caused. By getting rid of the natives, they solved both problems.
I think that the removal of Native Americans was facilitated on both grounds due to White individuals' appropriation of the world in accordance to their own subjectivity. As far back as the War of 1812, and even to the time of the Lewis and Clark expedition, the reality was that the frontier, or the idea of "what lies out there beyond the blue horizon." The construction of a social context where Native Americans were seen as "the other" that has to be appropriated in regards to control and political supremacy. In this sense, the idea of Manifest Destiny became a way to control, to seek out something that was legally unclaimed in the eyes of White America and take it for themselves. At the same time, post- Civil War industrialization that took place all over America became a realm whereby the overtaking of Native American land was not only seen as political reality, but furthering economic goals of commerce and the facilitation of expanding revenue streams. In both of these, the common denominator was the control and dominance of Native Americans by White Americans.
The reason for this is that white settlers believed (even in the case of the "Five Civilized Tribes") that Indian and white settlements were incompatible.
The whites believed that Indians had to be removed in order for whites to exploit the land properly. In the case of nomadic Indians, this was certainly true. Whites could not really farm and settle if Indian bands were constantly moving through the lands that the whites were settling. In the case of the more sedentary tribes, it was simply that the whites did not want competition from the Native Americans.
Since there was so much white demand for the Indians to be removed, removal became a political necessity. No politician could oppose Indian removal and hope to get the majority of the vote from the white people (who were, of course, the only ones allowed to vote).
So, whites wanted to exploit the Indians' land economically. This made Indian removal an economic necessity in their eyes and led to it being a political necessity for their representatives.