As a Ph.D. student, should I spend time reviewing papers? I am a PhD student halfway through my journey. After I published a few papers, I am getting review requests from some medium rank conference and journal editors. While certainly it is an honour for me, doing good review takes some time since I am not an expert of my broad field with everything at my finger tip. Does review experience add something to my CV particularly if I want an academic career? I am really sorry if the question sounds too shallow or cynical but I want to know whether I can cite the review experience to prove my worth as a researcher to a prospective hiring committee.
An important part of an academic career/CV is service. Reviewing, editing, hosting conferences, ... falls under this category. So yes, it is important.

You seem to consider it to be a potential waste of your precious time. This is false. Depending on your field, reviewers have earlier access to new work than others which is always beneficial.