I think it depends on what you do for work. I'm not going to ask an agent to book my flight for my vacation to French Polynesia. I want to pick my seat and potentially find a deal making an upgrade worth it, choose an airline, etc.
But my routine business trips in the CONUS with strictly defined booking options... let me just email an agent "Get there by meeting on day A, leave after meeting day B" and have it sort it all out without the drudgery of the corporate travel portal. YES PLEASE!