In CSS Lint, they don't recommend using an id for selecting elements. I trust CSS Lint because it written by smart people who know CSS very good and in depth. But I want to know what are reasons of this? Why is selecting with an id not a good thing?
CSSLint gives a guide to why they make their recommendations:
IDs shouldn't be used in selectors because these rules are too tightly coupled with the HTML and have no possibility of reuse. It's much preferred to use classes in selectors and then apply a class to an element in the page. Additionally, IDs impact your specificity and can lead to specificity wars.
(From Disallow IDs in selectors.)
Basically, if you structure your code to use classes rather than IDs, your code can be more general and reusable, which is generally a good thing. Furthermore, specificity is a hard thing to get your head around, and can cause bugs that are hard to find, so if you omit ID selectors, you're less likely to have conflicting rules resolved in unexpected ways.