Iris Data Services offers, among other things, a managed services partnership arrangement with law firms and legal departments which relieves the lawyers of responsibility for running and maintaining eDiscovery software, allowing them to concentrate on running their eDiscovery projects.
An article by Daniel Gold on the Iris blog is called How to change the way your firm thinks about eDiscovery. Inevitably (because that is what Iris does well) it promotes the benefits of the managed services route. Along the way, it sets out points for lawyers to consider when they face the broad choice between buying and running their own systems and delegating the technical and related functions to a company like Iris.
You will find similarly eloquent arguments elsewhere tending to the opposite conclusion. There is no fixed rule which applies to all firms in all circumstances. The important thing is to be aware of the implications of one approach rather than another so that an informed decision is made, appropriate to the firm, the client, and the case.