I am currently working on PHP sites (80% work is on heavy database queries) and also working with Zend Framework.
In my 2 yr development experience I never worked on any CMS like Wordpress, Magento but I have idea how they work.
Shall I continue with what I am doing or shall I switch myself towards CMS. Guide me which one is beneficial CMS vs. PHP and Zend Framework ?
I'd switch to CMS.
Look at Drupal, it is flexible, developer-firendly. Has extensions.
Then Joomla is not bad thing.
Using Drupal or other CMS that is in active development - you benefit from community contribution. A lots of smart people contribute to the project. A lots of new features appear every week. Forums, support.
Use a CMS if there's a well-maintained CMS that does what you need to do. For example, if all you need is a blog, just use WordPress and some plugins (and maybe customize it a little) rather than building a blog engine from scratch.
If what you're trying to do can't be done with an existing CMS + plugins, then it's time to pull out a framework.
Your question is very subjective, and there is no "right" answer, however, I will try to give you a bit of insight as best I can:
CMS systems are good when you need a quick website that is easy to use and manage. They are very flexible, and can be used in a variety of different situations. Most well-designed CMS systems are also easily expanded upon, making existing ones easy to customize to your needs. Even the underlying code can be modified (though you'll need to make sure you aren't breaking any licensing rules by doing this).
That being said, you'll find that sometimes it is harder, or impossible (in the case of licensing issues) to modify an existing CMS to your needs). For this reason, it's a good idea to see how CMS systems work, in case you are ever required to create one, but in effect, they are not very complicated. Most are just a collection of units, such as a log-in system, forums system (which is usually integrated with some other forum software), voting system, etc, which can be easily replicated (especially with the Zend Framework), and reused.
My advice to you is to at least understand how these systems work, in fact, down to the bare PHP (without a framework), so you can get a better picture of what is happening. However, evaluate each and every project you work on; if it is more time or cost beneficial to use an existing CMS, with or without modifications, then use that, otherwise, make sure you know how to create your own if it is needed.