Estimating in terms of units is mostly for internal teams with a fixed payroll. It can work really great in that context. But imagine that you are asking for project funding. The person across the table logically wonders: how much do you need? You yourself logically wonder: how much do I ask for? In that environment, can either party really accept “who knows” or “5.5 units” as the answer unless money is not a concern for anybody?
In my opinion, the estimate gives them a sense of scale. If some small number of months would cost too much, then you know and can start to have an investigation project of a week to better help define what should be in and what shouldn't.
