If what you do - raise the barrier to participation so high that hardly anyone is going to bother commenting or tagging - speaks louder than what you say - 'sure, we'd love to hear what you have to say' - which one do you think wins?
To pick an example I've seen recently (and this is not meant to be a criticism of them or their team because I have no idea what the reasons were) the London Transport Museum have put 'all Museum objects and stories on display in the new Museum' on their collections website, which is fantastic. If you look at a collection item, the page says, "Share a story with us - comment on this image", which sounds really open and inviting.
But, if you want to comment, they ask for a lot of information about you first - check this random example.
So, ok. There are lots of possible reasons for this. UK museums have to deal with the Data Protection Act, which might complicate things, and their interpretation of the DPA might mean they ask for more information rather than less and add that scary tick box.
Or maybe they think the requirement to give this information won't deter their audience. I'd imagine that London Transport Museum's specialist audiences won't be put off by a registration form - some of their users are literally trainspotters and at risk of believing a stereotype, if they can bear the kind of weather that requires anoraks, they're probably not put off by a form.
Or maybe they're trying to control spam (though email addresses are no barrier to spam, and it's easy to use Akismet or moderation to trap spam); or maybe it's a halfway house between letting go and keeping control; or maybe they're tweaking the form in response to usage and will lower the barriers if they're not getting many comments.
Or maybe it's because the user-generated content captured this way goes directly into their collection management system and they want to record some idea of the provenance of the data. From a post to the UK Museums Computer Group list:
We have just launched the London Transport Online Museum. Users can view[My emphasis in bold]
every object, gallery and label text on display in our new museum in Covent Garden.
Following on from the current discussion thread we have incorporated into this new site, the facility for users to leave us memories / stories on all objects on display. Rather than a Wiki submission these stories are made directly on the website and will be fed back into our collection management system. These submissions can be viewed by all users as soon as they have passed through moderation process.
We will closely monitor how many responses we get and feedback to the group.
Please have a look, and maybe even leave us a memory?
Moving on from the example of the London Transport Museum...
Whether the gap between their stated intentions and the apparent barriers to accepting user-generated content is the result of internal ambivalence about or resistance to user-generated content, concern about spam or 'bad data', or a belief that their specialist audiences will persist despite the barriers doesn't really make a difference; ultimately the intentionality matters less than the effect.
By raising the barrier to participation, aren't they ensuring that the casual audience remains exactly that - interested, but not fully engaged?
And as Seb pointed out, "Remembering that even tagging on the PHM collection - 15million views in 2007, 5 thousand tags . . . - and that is without requiring ANY form of login."
It also reminds me of what Peter Samis said at Museums and the Web in Montreal about engaging with museum visitors digitally: "We opened the door to let visitors in... then we left the room".
(If you're curious, the title is a reference to an Australian saying: Clayton's was "the drink you have when you're not having a drink", as as Wikipedia has it 'a compromise which satisfies no-one'. 'Ersatz' might be another word for it.)