With term limits in place, Congress will be more responsible toward their constituents because they will soon be constituents themselves. They will have to live under the laws they have created while in office.
Members of Congress will have less time in office to develop financially beneficial commitments to lobbyists and other special interest groups, thereby undermining the threat of lobbyists being a primary influence on legislation.
Our nation needs fresh ideas. Limiting the terms of congressmen and women will ensure that new ideas make their way into the legislature in greater numbers than they are today.
Power corrupts, and absolute power corrupts absolutely. Limiting congressional terms provides another limitation on the power of any given individual, after Democratic fashion.