Yesterday, I said I'd talk about why WordPress achieving 25% market share matters, and then I didn't do it like I intended. I talked more about the significance and challenges, and not as much why the market penetration matters. The real why it matters, I think, is because though the CMS market will always be contestable, WordPress does have a relative dominance of the CMS space. And that dominance is a powerful persuader to potential clients.
Due to WordPress's ubiquity as a tool-of-choice amongst development, advertising, and marketing agencies, it makes the software (in most situations) a safe and compelling option for a company. This is especially so once you get beyond the common and relatively easy to quell concerns potential clients often have regarding security and large scale capabilities.
Let's consider this scenario: a client has a large site build , a limited budget, few platform requirements, and they operate in a small to medium size market.
The client could receive bids for projects architected on a variety of platforms: WordPress, Drupal, Ruby on Rails, .NET, and many more. WordPress is a proven winner for timely projects on a budget; that's not the say it has to be just for time-limited or low budget projects, but it can definitely operate in that environment, whereas some cannot. Also, in a small market (say a city of < 250,000 people, or even more significant in a city of < 50,000 people), one or two agencies may do Ruby on Rails or .NET work, but nearly all of them probably have experience with WordPress, even if it's not their personal corporate preference.
That means that the client can change providers more easily. Now, I'm not trying to say WordPress makes firing your developer easier, but it does. It's more important for if a client outgrows a current service provider. Also, with WordPress, starting small (and cheaper), is possible; whereas with many other platforms there is sometimes a significant project floor.
WordPress's market penetration makes it a more compelling choice for businesses trying to make a decision, and is icing on the cake for those folks pitching WordPress, considering there are many reasons to choose it even without that market position.