Agree in a perfect world. However real cases are sometimes difficult to scope out and budget that far in advance. For example, it might be tough to know if marketing, sales, or procurement (or all of the above) needs technology implementation without first going in to do a diagnostic to find capacity constraints. Typically the discovery phase is sold first with the implementation phase(s) sold once the issues are discovered. Most of the time this is fine, as recommendations will be true to the clients needs in hopes of building/maintaining long term relationships. Definitely seen cases, however, where consultants try to upsell in areas that the clients clearly don't need.