A metal detector is often useful for locating benchmark disks, particularly when the nearby reference objects are no longer there to measure from. It can save a lot of probing.
Very small and cheap detectors are adequate for many situations. If you are going to look for a disk on a mountain top, it may be worthwhile to carry that lightweight and not-very-sensitive detector to scan for a disk under lichen, dirt, or leaves.
There are other times when a detector with the best discrimination between metals is vital, such as when looking for a disk alongside the recreation trail that used to be a railroad. There is enough scattered iron hardware, and sometimes iron-bearing slag used as ballast, along any present or former railroad to keep a metal detector busy all day. A detector that can ignore iron and respond to more conductive metals such as the bronze of a benchmark disk is necessary there.
For most situations, you want a "treasure hunter" type of detector because these respond to the conductive surface of a metallic object. Another type of detector is the magnetic locator (most famous brand is Schonstedt) which is carried by surveyors to look for iron stakes. These detectors ignore a bronze benchmark disk. Since very few benchmarks are on iron rods (mainly 1960's or 1970's USGS) this type of expensive detector is not what most benchmark hunters should carry.
You should practice at home with a variety of iron and brass, bronze, or copper objects so you are familiar with the behavior of your detector before going into the field.