At the last SaaS I worked at, it was absolutely true. Companies would sign large enterprise contracts, and then ~20% of their users would use it once and a while, and one to five users would use it religiously. It was a simple productivity tool, but some people really loved it. Most people didn't care or want it though.

The people that used it were usually the ones responsible for selling the idea to their company. I suspect that it was often a way to be able to boast increasing company performance so they could use it at a performance review or something. This was at a time when money was being thrown around like crazy in the VC world and they were all spending money like it was nothing.

The Pareto distribution in customers is probably very common in general. I'd just make sure you have good metrics/monitoring so you can see usage trends and make adjustments and optimizations appropriately. If the math works that you can offer it as such, that's great. The most expensive thing (probably money and time wise) is support. Some customers just need a lot more support than others. Sometimes for good reasons, sometimes not. Getting an idea of the common support requests and making sure they're easy to fulfill or having a pre-canned answer/FAQ is a good way to reduce that.