Are there any good reasons for disabling hardware-assisted virtualization?

The reason Dell (and Sony etc.) disable Intel-VT and AMD-V is that they cannot support it. Enabling the feature would mean they would have to provide support on it, which the simply cannot do, due to insufficient knowledge at the supportdesk, mainly.

That is, at least, how Sony formulated it.

I tried prying the reason from Sony support guys and that is the only thing they would give me. I finally was able to patch my BIOS and enable VT myself, though.

As for the rest, stuff like Bluepill are not exactly mainstream. And as far as I know - and I work with virtualization stuff a lot - there is no downside to enabling it. If there is though, I would really like to know about it...

One very good reason is security. There have been known hacks that insert a malicious hypervisor in between your OS and your hardware. This allows anyone to capture any data in a perfectly transparent manner.