I agree with this but I was more thinking about the jump from low to high fidelity mockups for the MVP landing page scenario where there isn't strong branding yet and multiple phases of user testing for many design choices isn't in budget either. Even when the branding is strong, it's still useful to brainstorm with the client over work in progress for early feedback too.
As an aside, what do you do when clients are resistant to doing user testing?
I think it needs to be right up there in your pitch from the start.
Position yourself as an “evidence based” supplier.
But for current clients, loss aversion is always the best way to get people to change their behaviour. What I mean by that is you have to quantify the value of lost customers. Sometimes that means doing some user testing anyway (perhaps on some other area of their product) and showing how many people are getting frustrated and/or giving up. User testing doesn’t have to be expensive, just find 3 representative people (even if one is your SO or the boss’s PA) and sit with them while they use the product. If you can tell a client this is costing you x million a year, and fixing it only costs x thousand then you’re pretty much there.
But like I said you’ve got to lead with evidence based approaches to get the clients you want.
As an aside, what do you do when clients are resistant to doing user testing?