There are a couple of good reasons why you should not root Android devices-
1. Say Good-Bye to the Warranty
As we all know, manufacturers are not keen on honoring warranty claims for devices modified by their customers. That includes both hardware and software alterations, the latter of which includes root. Except for a few companies (Google with its Nexus lineup springs to mind), most Android makers will void the warranty on your device, if it has been rooted.
2. Can Cause Update Issues
There are two main issues related to rooting Android and performing updates. First, after applying the update the root will most likely be removed and users will have to go through the risky process again. And two, updates may fail to install due to software modifications that occurred while the distribution has been rooted (either because of user alterations or due to the root access itself). So where does this leave you?
Well, in the first case the worst that could happen is that you will be unable to root Android, because the update patches the security exploit that was used for rooting. Needless to say, your low-level modifications will most likely be returned to stock as well, which can lead to a time-consuming process of having to revert (basically, re-do) them to their altered state.
In the second case, your device is basically exposed to any security exploits which would have otherwise been patched by installing the available update. It's a lose-lose scenario, either way, in most cases.
3. Rooting Is Not Easy Nor Trouble-Free
Experienced Android users may find the process of rooting Android as simple as installing an app from Google Play. Beginners, however, may find themselves in a pickle, having to deal with some not very straightforward procedures. It is not unheard of for a rooting process to fail, rendering a device useless (known as bricking). Luckily, in most cases a firmware flash will solve the problem, but this is not easy to perform either for beginners and will only lead to more downtime.
Similarly, when rooting users basically have to have blind faith in the developer responsible for the tool or the app that is/will be used, hoping that there's no sneaky backdoor built into it or that it will not harm the device. In most cases I haven't found this to be of issue (though one can never know for sure), but considering the multitude of Android devices available and the multitude of tools and apps available for rooting their distributions, the aforementioned risks are not part of a far-fetched scenario.
This article is not meant to scare you into believing that rooting Android is utterly useless and full of risks.
