When most of us think of "mentoring," the benefits that come immediately to mind are those accruing to both the protege and the mentor.
We know that the protege learns very useful information, about the firm, about the industry, about processes and procedures, about him/herself and other things. And we know that the mentor gets to sharpen knowledge that has perhaps become dull over the years -- it is a proven fact that teaching is one of the best ways to learn (or relearn) something.
And certainly staff members -- of any age and at any level of experience -- who have their knowledge and skills sharpened are a good asset for any company. But there's another benefit that accrues to the company -- a benefit which might be more important than those derived second-hand from benefits to the protete and/or the mentor.
Mentoring is a great way for the company to preserve and revitalize "institutional knowledge."
When someone retires, passes away or takes a new position elsewhere, the company generally loses the knowledge that was stored in that person's head. It's a shame, but it's also a fact of business life. However, a mentor/protege program -- whether formal or informal -- can mitigate that loss. It doesn't matter whether the knowledege is factual or anecdotal; it is of vital -- often critical -- importance that the firm retain that knowledge.
If all of your senior technical staff take junior members under their wing -- talk to them about why processes and procedures are the way they are; tell them about what was tried and why it succeeded or failed; give them knowledge of what the client for each project likes and dislikes; teach them how the firm, the industry and the business world work; and guide their acquisition of new knowledge that would be relevant to their jobs and their imagined futures -- then much of that institutional knowledge will be passed on to the next generation of leaders before the senior members depart.
It may turn out that very few things are as important to the firm as this!