Query: Why pay for end-of-day historical prices?

Relevant passages:
 <=SEP=> "There are several reasons to pay for data instead of using Yahoo Finance, although these reasons don't necessarily apply to you if you're only planning to use the data for personal use.  Yahoo will throttle you if you attempt to download too much data in a short time period. You can opt to use the Yahoo Query Language (YQL), which does provide another interface to their financial data apart from simply downloading the CSV files. Although the rate limit is higher for YQL, you may still run into it. An API that a paid data provider exposes will likely have higher thresholds.  Although the reliability varies throughout the site, Yahoo Finance isn't considered the most reliable of sources. You can't beat free, of course, but at least for research purposes, the Center for Research in Security Prices (CRSP) at UChicago and Wharton is considered the gold standard. On the commercial side, data providers like eSignal, Bloomberg, Reuters also enjoy widespread popularity. Although both the output from YQL and Yahoo's current CSV output are fairly standard, they won't necessarily remain that way. A commercial API is basically a contract with the data provider that they won't change the format without significant prior notice, but it's reasonable to assume that if Yahoo wanted to, they could make minor changes to the format and break many commercial applications. A change in Yahoo's format would likely break many sites or applications too, but their terms of use do state that Yahoo ""may change, suspend, or discontinue any aspect of the Yahoo! Finance Modules at any time, including the availability of any Yahoo! Finance Modules. Yahoo! may also impose limits on certain features and services or restrict your access to parts or all of the Yahoo! Finance Modules or the Yahoo! Web site without notice or liability."" If you're designing a commercial application, a paid provider will probably provide technical support for their API.  According to Yahoo Finance's license terms, you can't use the data in a commercial application unless you specifically use their ""badges"" (whatever those are). See here. In this post, a Yahoo employee states: The Finance TOS is fairly specific. Redistribution of data is only   allowed if you are using the badges the team has created. Otherwise, you can use YQL or whatever method to obtain data for   personal use. The license itself states that you may not: sell, lease, or sublicense the Yahoo! Finance Modules or access   thereto or derive income from the use or provision of the Yahoo!   Finance Modules, whether for direct commercial or monetary gain or   otherwise, without Yahoo!'s prior, express, written permission In short, for personal use, Yahoo Finance is more than adequate. For research or commercial purposes, a data provider is a better option. Furthermore, many commercial applications require more data than Yahoo provides, e.g. tick-by-tick data for equities, derivatives, futures, data on mergers, etc., which a paid data source will likely provide. Yahoo is also known for inaccuracies in its financial statements; I can't find any examples at the moment, but I had a professor who enjoyed pointing out flaws in the 10K's that he had come across. I've always assumed this is because the data were manually entered, although I would assume EDGAR has some method for automatic retrieval. If you want data that are guaranteed to be accurate, or at least have a support contract associated with them so you know who to bother if it isn't, you'll need to pay for it."