Why isn't Suspend and Resume enabled by default
This is actually a complex answer, but simply put only machines that work should have the feature enabled.
What we then fall into, is "How do we know it works?"
There are lots of things that can cause suspend/resume to fail: the previously mentioned driver support, funky BIOS's, bugs, and untested edge conditions.
Due to the fact that there isn't a way for drivers to tell the OS that they support suspend/resume, the only way to check that is to try. The good news, is that if a driver doesn't support suspend/resume, it fails gracefully and returns the machine to it's pre-suspended state. This does become a problem, though if the driver does support suspend/resume, but fails due to a unexpected change in the hardware that the driver doesn't know it needs to support. And if this occurs on resume, the only recovery method is a panic and restart. And even if the hardware is as expected, the user might be using it in some unique configuration that hasn't been tested. Should these problems occur without user intervention, then it would be considered to be far worse than if the user turned a on a knob.
So the way it is approached in OpenSolaris is to select a set of machines that can be tested as extensively as possible (even get the vendor involved in this testing) and have those enabled, such that though some of the above problems might still exist, there is some reasonable likelihood of solving problems should they occur. For others, we basically are asking that to use the knob, and if it works for them, let us know the details. If it doesn't, we can get to it as resources become available (and as this is OpenSolaris, the users could help solve problems).
Even though it is an extra step for many, at least they know how to turn it on, and even off, should suspend and resume not work for them.