This question seems to be aimed at two different things a) what is possible and b) what has commonly happened.
Its quite correct that electric propulsion has been used for all of the following purposes:
- Life extension
- Launch mass reduction
- Flying more payload mass for the same launch mass and lifetime
There are no general rules to this and it is certainly not true that mass saving has been more common than payload maximisation or life extension. There are only a couple of examples of clear mass reduction and the large bulk of previous missions are rather more mixed.
The following examples address each of these points and in some cases its hard to tell which of the purposes was adopted as the driving reason to use electric propulsion (EP). By the way, for that term we can include gridded ion thrusters, hall effect thrusters and also hydrazine arcjets and resistojets.
For the last fifteen years or more a 15 year life has been the standard adopted for most commercial geostationary communications satellite. A key life-limiting feature is inclination drift and so satellites with EP for North South station-keeping often have a larger propellant life margin than those with chemical propulsion only, often of the order of 5 - 15 years over the initial 15. i.e. there really are satellites with 25-30 year propellant lives! Recently longer design lifetimes have begun to appear as the baseline mission length, though typically this has only crept up a year or so from 15.
The mass reduction argument has changed recently. For NSSK over the last decades it depends upon the EP type. The satellites that have gridded ion engines and hall effect EP for NSSK only have tended to be the larger ones, in the range 4 - 6 tonnes. The point is that the overhead of using the higher Isp systems for NSSK is such that its more worthwhile for larger satellites. Satellites using Arcjets or Resistojets have been smaller.
In the last couple of years, with the introduction of EP for GTO-GEO transfer there have been different flight examples. ABS 3A and Eutelsat 115 West B achieved a given payload at a small mass to get on a dual launch on a Falcon 9. Viasat 2 appears to have gone for a maximum payload.