Beginning with code version H.25.3, Omniture introduced a new cookie that will serve as a fallback method in case your third-party cookies are not accepted.
Can anyone tell me if there are any drawbacks when using this method? Why would someone use the first party cookie implementation instead?
I think that the following information answers my question. They come straight from Adobe 1.
Visitor accuracy matches true first party cookies: This solution sets a first party cookie and its rejection rate is as low as any first party cookie implementation.
Visitor Uniqueness: The methodology used to generate the s_fid value does not guarantee a unique value for each visitor. Factors that impact uniqueness include the volume of s_fid values set across the site and the number of concurrent hits from visitors who arrive at the same millisecond. It’s important to note that the odds of this are very low, and you shouldn’t notice an even remotely significant difference between visitor counts using this methodology and using CNAME first-party cookies.
Decrease in visitors after code update: If you update your analytics code to H.25.3 or higher, you may see an overall decrease in your visitor count because of the decrease in overall cookie rejection.
http://blogs.adobe.com/digitalmarketing/analytics/want-a-cookie/