I think that OP has this wrong. It's not the TA which gives the discount, but BA. However, one of the conditions of almost all corporate discounts -- which can be quite substantial -- is that the business use a qualified TA to handle all bookings.

This also allows the employer much greater control of bookings and substantial savings in accounting (because most business TA's use software which makes transfering the costs into the company's bookings much easier.

Downsides are: 1) general loss of control of travel; 2) UG's.