The title deed is the official proof of ownership. However there are thousands upon thousands of property owners who have never applied for theirs.
Before the Oqood registration was implemented, many owners who bought a property off plan only had a Sales Purchase Agreement (SPA). Contained within the SPA you had the sales agreement terms, the buyers name(s) and developer/seller(s) name with every page bearing the developers stamp. This document is sufficient proof of ownership to allow you to rent the apartment, connect DEWA (Water and Electricity) and DU/Etisalat (TV, internet and phone).
Ideally their government wants you to apply for the title deed so that they can make their 4%, and in the past have bought in schemes which been designed to try and force people into applying for their title deed. Personally I would advise that you do apply for it as it is the only official document which proves ownership and therefore is very valuable should anything happen in the future.
In reality the only time you would require the title deed is if you were to sell the property or apply for a visa on the back of that property.