The downside of hiring specialists to supplant outside counsel are several. When you add a lawyer skilled in an area of law you risk (1) that the specialist work might dry up, (2) the compensation paid will upset internal equity, and (3) turnover when the economy strengthens. Law firms are more attracted to specialists than corporate generalists. You also incur more administrative time because of another staff member and you increase your employee costs, which are fixed, as compared to your outside counsel costs, which are variable.
Advantages of a specialist hire, however, are evident. You should be able to reduce the amount spent on outside counsel in that area of law. The lawyer under your roof is also both more subject to your direction and oversight and better positioned to get to know the business and industry.