I watched some-one (retired 744 skipper) fail an airline Sim assessment in the 733 'cos he left the A/T in whilst manually flying. The speed was +/- 10-15kts and the altitude +/- 300ft. It was painful to watch him struggling with a not very helpful (management) co-pilot. When it was my turn, I took the A/T out and passed!
O.K. it was only a sim but I certainly think it destabilises you slightly unless it is in a thrust mode, as noted previously.
We teach the same on the 744, by the way.
Qavion, the 733/734 A/T will retard automatically at 27ft RA (from memory), even on a manually flown approach.
A merry Xmas to all my readers! (O.K. my reader!)