My thoughts are, as someone who worked from the consulting end and not management or sales, that when you hire sales people to bring in consulting work, they inevitably bring in commoditized work because sales people have unique incentives and pressures. If you are at the point where you are trying to get sales people to sell specialized consulting work, you aren't doing it correctly. It more or less has to sell itself with a brand as you say. Sales people don't want to spend the time building relationships or learning specialized information to sell to specific targets. So you can't scale up a small consultancy imo.
From the employee side, it is boring and meaningless work that burns you out.
I've seen big consultancies buy up small boutique consultancies and it always turns out badly. My own thought is if you are an IT consultant and you find yourself doing commoditized work, something has gone wrong.
I spent a lot of time in consultancies and started one myself, I'm not tptacek but here's my two cents.
Does the buyer see you as a commodity? How hard is it to find someone else to do what you do?(as defined by the customer not you) Can you differentiate yourself from competitors?
I think it really comes down to establishing a brand. And the problem is if you can't establish a brand you can't charge enough to fund growth (and at the beginning charge enough to give you time to grow the company).
A company with a brand can charge 2-3x their labor costs. A company without one can maybe charge 25% more than their labor costs and this puts you in the classic consultancy spiral where you spend so much time working for your business you don't have enough time to work on your business.