I know it is tough to spend time to prepare for a certification when you are tightly engaged with assignments. There is a big difference in achieving knowledge from technologies/frameworks/tools experience and taking certifications.
Certification objectives may not cover the whole technology/API and it adheres to property standards. For example, Java JSP and Servlet certification tests candidate knowledge in Java EE web technology and frameworks recommended by Oracle and you may not gain knowledge like struts or any other web frameworks.
Assignment experience using technologies/frameworks/tools may not cover complete platform technologies for APIs. For example, candidate working in Java EE JSP development may not have experience in business tier development (EJB) and persistence tier development (JPA).
Certification knowledge empowers your knowledge to decide which technologies/frameworks/tools to choose for your business problem.
The technologies/frameworks/tools experience boosts your working knowledge to apply certification skills (the knowledge you achieved during exam preparation). Note that frameworks/tools are derived from standard platforms (Java SE /Java EE/ Java ME), it is always good to know the platform first.
Both certification and technologies/frameworks/tools experience adds value to your career profile.