> Is it routine for historical stock price data to be revised?

Yes. In fact, this happens not just for mergers, but also for dividends, stock splits, and spinoffs.

> Excluding splits, are there any other events by which historical data may be altered?

The most common occurrence is when a company pays a cash dividend. If you are looking at a dividend-adjusted price chart, then its historical prices have been adjusted to remove the sudden drops in prices on the ex-dividend dates.

> Who benefits by this alteration of historical data?

There are uses for raw prices (unaltered data), but for the vast majority of people who use price charts for analysis, the adjusted data is more useful because it makes the current data more comparable to the past data. For example, if a company does a large spinoff, its stock price will decrease. If you only look at the raw prices, it would seem that the investors lost a lot of money from the "crash". The price adjustment is meant to present a more accurate picture of investment performance.

Formulas used for adjustments:

* [CRSP Calculations](https://www.crsp.org/products/documentation/crsp-calculations)
* [Nasdaq Data Link Blog: The Comprehensive Guide to Stock Price Calculation](https://blog.data.nasdaq.com/the-comprehensive-guide-to-stock-price-calculation)