Pre-sales support also is a factor. When the engineer goes on site, conference call, or any other means of pre-sales support for/with the sales guy, this is a value add as well. Any help to the sales guy is a value add. The engineer can explain the "whys" and "why nots" to a customer that helps the customer make a good decision. If the customer decides to buy the equipment/services, then the engineer was certainly a value add to the sales guy. We call this pre-sales support, and this is valuable to the company.
How about an engineer's skill set? Does the engineer have a valuable skill set? Does he have a lot of desired skills or maybe just one? The more skill set the engineer has, the more valuable he is to the company. Plain and simple.
How about the mentoring of the other engineers in the company? When a senior engineer helps/mentors another engineer, wouldn't you think this is a value add for the company? I mean, you teach someone a skill, or even how to consult. It could be anything. You make other engineers better by sharing knowledge, and this makes not only those engineers more valuable, but also increases the value of the company to a customer, who gets more value out of your engineers. This is a good thing, and a value add to the company.
How about when an engineer is very busy, and provides more work for other engineers by sharing his load? Sometimes when other engineers are not busy and you are, you boost their value by sharing your work load. They become billable and this benefits the customer by getting things done in a more timely manner. It also benefits the company you work for by the engineer not sitting idle. This is a value add to the company.
There is more value to an engineer than just his billable role in the company. Some of the things above, I don't think you can put a financial number on. But either way, they are valuable to an employer financially speaking. Don't let someone tell you how much you are worth. You can determine that for yourself.