This is basically a follow up for this message on Twitter: https://twitter.com/ZenphotoCMS/status/598549642553491456
140 letters is a bit too short to reply, so I prefer to do it here.
On my Zenphoto Theme, Paradigm (http://www.france-in-photos.com/pages/zenphoto-theme), I changed the way tag links are generated, basically removing the "nofollow" on these links.
Doing this allows search engines to index this kind of page: http://www.france-in-photos.com/page/search/tags/sunset
The idea is to provide search engines with numerous pages highlighting specific keywords. Those keywords can be different from the ones used elsewhere in the website: in gallery titles for instance. It is a quick way to provide more keyword coverage. It is easily done if you manage your tags in Lightroom and export them to Zenphoto.
I have to say this not work very well on my website: tag pages are not optimized enough to be really effective. The major landing pages for search engines are the albums, then the homepage and individual images. A tag description field (like in WordPress, but in rich text) could make these pages more effective.
The second aim is to give alternative paths to find image pages. In large galleries (1000+ images), the tag pages act as "thematic hubs" and distribute links in an alternative way. This may reduce the number of steps search engines have to take to discover your whole structure, though it also adds lots of new pages for them.
On my website, the gain is probably one depth level.
Tag clouds are also effective for this too, though on large galleries, they require lots of calculations and slow down page loading (higher time to first byte). This is why I limited the use of the tag cloud to search pages in the latest version of my theme.