What are the side effects of disable a core module? (e.g. Mage_Rss / Mage_Log)

Aside from the obvious, e.g. the module no longer working - a potential side-effect is any module that has that core module listed in their declaration would also not function. There are many 3rd-party plugins that depend on core modules loading. There are core interdependencies, too, especially if you're using Enterprise.

I'm under the impression that most people disable these 'unnecessary' modules for speed purposes - after all, only load what you use, right? While I'm sure it has some small, measurable impact - there are much more effective things you can do to positively impact the speed of your site.

There are some great SO articles out there that talk about this particular issue - this one in particular seemed like a good read to me:

Speed Optimization by disabling Mage_Reports module?

There are other reports out there that disabling Mage_Downloadable, etc. may help in Checkout - but these seem to be aging posts and aren't relevant in my opinion to the latest releases of Magento. If you're still running on 1.4CE or 1.7EE, this might be particularly useful to you.

http://kassner.com.br/2011/01/magento-slow/

http://www.magentocommerce.com/boards/v/viewthread/221626/#t306872

https://stackoverflow.com/questions/11102268/magento-duplicate-orders

Update

It's hard to generalize what measureable impact this would have for your particular - or anyone's - situation. A slow-ish server on a low-bandwidth network, for instance, would impede your speed more greatly than any of the mentioned solutions.

Based on your own intensive analysis it seems it may profit you. As with all SO feedback, YMMV, but I think that using memcached, apc, full page cache and a CDN can have a more measurable impact to something like, say, conversion rate as it impacts the end-user experience.

I configure all client boxes based on the published whitepaper specs - to my knowledge they do not make mention of disabling unused modules. They do make mention of disabling unnecessary Apache Modules.

Whitepaper

http://www.magentocommerce.com/whitepaper/

Lastly, I have done some digging and can only find mentions of disabling Mage_Rss as a potential performance boost when using filesystem cache. If you're using filesystem cache, and haven't at the very least mounted that to tmpfs, you should seriously consider memcached, apc, or the aforementioned tmpfs mount.

We've seen a specific issue with disabling Mage_Log that it breaks Product Compare functionality. When anonymous browsers add to compare and then log in, Magento uses the visitor_id generated by Mage_Log to attach the compare list to the customer, and if this value is null then it breaks the site, hard, if any other customer has ever compared the same product_id (uncaught duplicate key exception).