How bad is it to not dispose() in Powershell?

This has been edited to include a safe, nonspecific answer.

IN GENERAL: dispose everything, because Dispose is the .NET framework's way to free up external resources (such as file handles, TCP ports, database connections, etc). Resources are not guaranteed to be released unless you call Dispose(). So beware. This is the general, non-SharePoint answer.

SPECIFICALLY WHEN DEALING WITH SharePoint:When you close the PowerShell.exe process, the memory is freed. If you need to dispose objects to keep memory pressure down (important in production environments or if you're looping over all sites/webs), make sure to dispose. If not, you don't need to worry about disposing.

The reason why we're so crazy about disposing in the first place is because most SharePoint code runs in long-running processes (either in an ASP.NET worker process or OWSTimer.exe) and failing to dispose can cause difficult-to-troubleshoot, sudden catastrophes (i.e., web server go boom). These catastrophic performance issues/OutOfMemoryExceptions don't affect me most of the time when working in PowerShell. I run ad-hoc scripts, I waste ~3-50MB of RAM because I fail to dispose my objects, I close the PowerShell window and the memory is freed. Most of the time it's a nonissue.

I've built scripts for working with SharePoint, and most of the time I don't bother disposing.

Here is a script wherein I dispose SPSite and SPWeb objects

Here is a script in which I don't bother to dispose an SPSite object

Ideally, you should call Dispose when possible. Even inside of PowerShell.

The SharePoint libraries include a lot of code that is just a wrapper around COM objects - and to make life more fun, many of those COM objects have their own pools and caches. The .NET call to Dispose really just instructs the COM objects that they can free their own objects (that may have a lifespan outside of the calling process).

Here is some more relevant info: http://blogs.msdn.com/sharepoint/archive/2009/02/11/sharepoint-and-powershell-knowledge.aspx

Just follow this.

Even if it's a simple script that frees memory once executed, you never know whether at some point that gonna be copy/pasted into an inner loop of a bigger script :-)

For correctness, you should always dispose SP objects as specified in the link above.