Unfair to whom?
The major advantage (to the company) in using a Convertible Promissory Note is that you can "postpone" the issue of "valuation" until a later date.
You are accurate when you say that a ultimately the valuation of a business (especially pre-revenue) requires 2 parties to agree on it (the buyer/investor and the seller/owner). I certainly wouldn`t spend money to get a 3rd party valuation at this point. However, it`s not just a guess and not really a good idea to just "pick a number" - there are some sound processes to follow that will give you a range which you can use to begin talks.
Although the Convertible promissory note is a viable tool, it has does create company debt, which may commit you to another funding round in the near future.