I’m generally a big fan of open source Projects, I have scraped some sort of living using systems like WordPress and Drupal over the last 10 years. I have added some of my own bits of code to some projects and used lots of other peoples code too. That is essentially how it works, by designers and developers contributing to a projects codebase to hopefully making things better.
It has always been interesting to read some agencies/developers impassioned arguments for and against various systems or open source itself. I noticed a while back that Avaaz moved off the Drupal platform to what I can only presume is something totally custom built. That seems to me like the right scenario, a good open source project like Drupal can play an important part in an organisations growth, it may not be perfect, but, for the relative costs it is probably pretty darn close. Then when the time and money are right, getting a custom solution built makes sense.
The wrong way to do it is spend all your budget on a custom system, only to struggle with maintenance costs, updates and changes of emphasis. It can also leave companies completely reliant on one Development agency who can then charge as much as they like.
I don’t only use open source, for example, I’m a big fan of Shopify which allows me a decent amount of flexibility in terms of building websites, but, manages all the less intriguing things like software and server updates and management. It is what is called Saas (Software as a service), usually pay monthly with no commitments and currently it is pretty good value.