Monocultures are bad: the dominance of Microsoft Windows and Microsoft Office is a great example of this, often cited by Open Source advocates.
One system in use almost everywhere means that one common set of weaknesses is also almost everywhere, and other systems/cultures shape themselves to depend on the monoculture. Monocultures are stable in the short term but prone to sudden collapse, causing much wider damage to dependant neighbours. Variety, adaptability and competition are better: for genes, societies and software.
This is why the fantastic success of Apache is beginning to worry me. According to Netcraft’s recent survey of web servers, almost 70% of active web sites are provided by Apache. Companies are (understandably) dumping Microsoft’s IIS, and most are then moving to Apache.
I like Apache - it’s well documented, powerful, flexible, reliable and truly free software. The Apache project is a model of good open source development. But it’s still becoming a monoculture, and even with its excellent reputation for security, a major flaw in Apache could cause immense problems for the Internet. We need more heavyweight web servers - most alternatives to Apache are either specialised or lightweight, and once you’ve enjoyed Apache’s feature set it’s difficult to change to much more limited software.
I almost wish IIS was better.