One of my pet annonyances when browsing around on the net is sites where you have to register for no good reason. I have enough useless accounts as it is. What's even more annonying is when they return a different result to the search engine bots so that more than just the registration page is indexed.
A prime example of this is Unison.ie. When searching for current Irish news it usually ranks fairly high on Google, however all the pages require you register first before you view them. The registration gives no advantage to people like me who just want to a quick look at the latest news. I suspect that I'm not alone and that lots of people will just go back and look for another site.
Unison's simple user agent checking makes it very easy to get in unmolested though. The User Agent Switcher Plugin for Firefox allows you to easily set exactly what user agent you want your browser to appear as. The GoogleBot isn't in the list of Useragents available, but it is easily added. Switch to GoogleBot as your useragent, and magically you will have full access to the Unison site.
I know that Unison will probably close this hole within a few days now, but it's nice to be able to make a point. According to Google's Webmaster Help Center "crawler only" pages are a thing to avoid. I would class pages that react differently to GoogleBot as "crawler only" pages.
If Unison want to require people to register in order to get nice features such as customization, then grand, I have no problem with that. However, how much traffic are they missing out on by having the register page for everyone? And how many advertising impressions are they missing out on? I know that if I go to the BBC News site I will usually end up going to other stories which interest me, which means more page impressions on the BBC site. More impressions, more chance of clicking on ads, more money!
In this day and age it is senseless to have such stupid restrictions on a site like Unison that has enough content to be a massive earner on advertisments alone.
Update: I somehow managed to forget the user agent I'm using, it is: