Would you ever run 2 SharePoint environments in parallel? Why?

I would think there are loads of reasons, and with SharePoint 2010's "Service Application" architecture this becomes even more viable, where you can shared service between your SharePoint environments, such as central search / user profile services.

In terms of why you might do this, as I said there are several reasons:

The main reason would be for isolation, in terms of either security or functionality.

You may decide for regulatory reasons that all client/financial data needs to be kept on specific dedicated (and physically isolated) environments, which is expensive and not required for your other day-to-day run-of-the-mill usage?

In terms of a logical functional split for functionality; you might have a core central "Social Networking" instance, which runs your User Profiles, My Sites, activity feeds and perhaps some centralised social features such as blogs and discussions, and some social based sites.

Another SharePoint environment would perhaps be used for corporate document and records management, with the majority of users collaborating in document libraries, and perhaps incorporate some custom workflow, routing rules and using browser based Office Web Applications?

You might have another SharePoint environment which serves hardcore Business Intelligence modelling from your core Sales and ERP databases, surfacing KPIs and reporting to your executives, sales and operations teams.

All of these three environments would have their own distinct challenges. They would have different hardware requirements, be used by different users (and different licensing SKUs as well, such as BI using Enterprise features).

You may also have completely different SLAs in terms of backup and recovery .. or have a specific requirement to host your BI environment very close to your Data Warehouse.

Other reasons might be for globalisation / localisation; providing a core "global" publishing site for news and corporate templates, but then localised regional "satellite" installations for users to work on documents and collaborate day to day (I have seen this specifically implemented for client offices who work on large files, such as CAD drawings).

The good news is that SharePoint supports all of these different configurations .. SharePoint is flexible and meets most business needs around this. If you happen to have SUCH a strict requirement around data (such as financial transactions or military grade data) then you won't be looking at SharePoint anyway .. you will likely be using bespoke applications developed using custom hardware (such as dedicated encryption cards)