I agree that the two younger characters (played by James Read and Janet DeMay - and yes, I had to look up their names) were colorless, and replacing them with Doris Roberts strengthened the show.
Having said that, I really liked the first season episodes in which Steele would relate whatever case they were working on to some old movie, and then try to guide the case in the same way, often with disastrous results (e.g., "Double Indemnity, Fred MacMurray, Barbara Stanwyck, Paramount 1944, a man and a woman conspire to kill the woman's husband for insurance money...."). A favorite moment came on one of the early episodes when Steele pulls Laura aside and says "We're in the wrong movie!" They kept that up a bit in the second season but eventually dropped it.
I once saw an interview with Brosnan in which he said that he thought the show lost a lot when writer Glenn Gordon Caron left the show after the first season to start his own company (which eventually led to "Moonlighting," a show that owed a lot to "Steele").