Depends really on the band's age and current popularity- I mean, Yes were too inexperienced, in my opinion, when they did TAAW, and lacked enough media respect when they did Magnification. But generally, symphonic does work, especially with things like rock operas.
If we're talking individual orchestral instruments though, then that usually adds something positive. Flutes, violins, and to a lesser degree, brass, plus the percussion we mentioned. I think it probably works better within progressive rock though rather than straight/hard rock. Then again, there's people like the Beatles who got away with just about anything.
Which is better: rock groups covering classical music (using their rock instruments), or orchestral instruments adding a symphonic layer to a rock song/album?