The benefit of VSD centrifugal chillers will largely depend on its utilization range. If your application does have varying loads and also low ambient temperatures, it will suit VSD chillers. Remember that VSD chillers are efficient at part load with low ambient temperatures (cool condenser water temperatures). If you don't get these conditions, then the VSD chiller(s) run at near full load and you don't get the great efficiencies the chiller manufacturers tell you.
In multiple chiller plants, the chillers are typically fully loaded so there's little advantage for VSDs. Even if you do get low loads and low ambient conditions, chances are if it is a commercial application, you might have an airside economy cycle which limits your chiller operation anyway. No matter how efficient your chillers can be, you cannot beat free cooling!
It all boils down to the return on your investment: would you pay that extra for a chiller to have a VSD or would you use the same money to buy an even more efficient chiller ? e.g would you buy a chiller with VSD with 0.6 kW/ton or a constant speed chiller with 0.5 kW/ton if they were priced the same?
The answer lies in comprehensive analysis/simulation programs, one like VSSriram mentioned. Trane also has one called the System Analyzer which does similar comparisons. The answer is it depends...