Really, there are a few options:
1) Use a Theme that someone else maintains, and rely on the developer to make Theme improvements. (This includes using core-bundled Themes, such as Twenty Eleven.)
2) Modify a Theme, via Child Theme, and maintain only your Child Theme, while relying on the Parent Theme developer to make Theme improvements.
3) Fork a Theme, by modifying it directly, and then taking ownership of any future Theme improvements
4) Creating your own Theme from scratch, and maintaining it.
Pick whichever option suits your needs - but understand the impact of each decision.
You chose to modify Twenty Eleven directly, and are now concerned with maintaining changes to the Theme yourself. Okay, well... that really doesn't work.
If you're not comfortable with maintaining Theme improvements, then you should limit yourself to options 1 and 2. Thus, if you need to make changes to the Theme, but are not comfortable maintaining the entire Theme, you are left with option 2: use a Child Theme.
More often than not, a Child Theme is nothing more than CSS changes. Next most likely change is adding custom page templates, or customizing Theme template files.
The least likely change involves modifications in the functions.php file. If you don't have to make functional changes, your Child Theme doesn't even need to include functions.php. In such case, you have little to worry about when updating the Parent Theme.
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