Benefits from MLS werealleged to be ability to do multiple runway ends with one equipment installation, less site preparation (rearrangement of the landscape) than ILS. It would allow curved and segmented approaches which could allow some more flexability for constructing instrument approaches.
There is MLS and the is MLS. A system was built for the US military in the 1960s and 70s by I believe, Singer Librascope. that was designed to be easily deployable into new airfields/LZs t o setup and provide instrument type capabilty quickly and easily.
A variation on this system was used by Rocky Mountain Airways with DH-7s in Colorado to provide more reliable flights into ski country airports that ILS could no be installed at or used usefully.
The FAA started into buying about 200 systems from Hazeltine in the early 1980s. This was different than the military systems and met a set of SARPS/MOPS that a number of companies were using including Canadian MArconi and some folks in Europe. After building about 20 system the Hazeltine contract was terminated for a variety of reasons. Systems were installed at Manchester New Hampshire, Haily Idaho and some other locations. There are also some in Alaska, I believe.
After Hazeltine there was still demand by specific operators and airports with specific local conditions for MLS systems so a FAR Part 171 program (where an airport buys the system to FAA specifications, installs it and gets it running and then turns it over to FAA for care and feeding) was started and so far as I know still exists.
The problem with MLS, as alluded to by FWA NATCA was that it didn't provide sufficient operational benefit to users to justify the cost or equiping and operating it. The General Aviation community refused to equip their aircraft because of the expense and lack of more benefits and the GPS was coming along that promied to provide at least a non-precision capability. LORAN C was also popular in GA for a few years in the 1980s as a poor mans RNAV system. In the end MLS is a useful technology in a niche market.