Useful to have the choice of both.

For a simple BA-only itinerary, I'll book via BA direct for the Amex points and MFU ability.

But for complex itineraries, last minute changes, and other tricky stuff I wouldn't have a clue how to do, my TA is invaluable.

I suppose the biggest factors which determine whether a TA is useful is a) how complex your itineraries are, and b) how good your TA is. Our TA is outstanding - even though I think I'm relatively competent when it comes to getting good fares and smart routings, they really are a level above, and employ numerous TA tricks of the trade (fake returns etc) to handle complex routings.

During the ash cloud incident of April 2010, I think my TA had me confirmed on about ten different Europe - US flights. Admittedly, none actually ended up taking off and I still didn't make it....... but they have their uses