Why plan pricing is better than per user(unit) pricing

Not sure that makes sense as plan pricing would always cater for the higher expected number of users, so a company with 500 users would do well using a plan priced for 400 users but a company of 5 would face a higher cost.

Maybe a better model is for users over a set period rather than installs at any one time. So if for 3 months you had 50 installs, then the following you only had 40 you'd be billed for those numbers for each period.