The first problem with the “Marketing Contribution to Revenue” metric is that it somewhat ignores the other aspects of marketing’s role. In most companies I have seen, in addition to demand generation, marketing is also tasked with brand development, product positioning, competitive analysis, sales training, support, and sales cycle support once sales has engaged a lead.
Even in the case of zero marketing lead generation, these activities contribute to overall corporate revenue. Take the case of a company where 
there is no marketing driven lead generation. In this case, assume all B2B efforts are done through cold calling. Even in this scenario, marketing contributes to revenue. To start a cold call, you need a trained sales rep (marketing trained). For a prospect to engage, they are going to look at a marketing created and maintained website. The prospect may also search for third party validation that marketing has obtained by working with press, analysts and other influencers.
Once a sales cycle has started, the sales rep will use marketing created scripts, white papers, ROI analysis and other material to move the prospect through the sales cycle.Finally, when it comes to close the deal, customer references and other key closing documents were probably created and maintained by the marketing team.
Measuring marketing contribution to revenue by the narrow definition of lead to opportunity closing, ignores significant value added activities done in marketing that support revenue generation. Gartner’s transit map below gives a good indication of the complexity. You can decode the image above here.
StoryMetrix 