Warrantless Wiretapping is the act of covertly overseeing communication without legal formalities in place. Such wiretapping is obviously the preferred method for any government to maintain order, as there is no pesky paper trail which might be used against the government later in a legal action.
Any government which has the appropriate control over its citizenry should never be called upon to justify its actions. The mere mention of the supposed desirability of a warrant is a very reliable sign that someone (perhaps several "someones" or even entire cities or states) needs to be disappeared. In fact, pathetic requests for such written justifications are telltale indicators that the citizenry is going rogue, and preventive measures cannot be too sudden or too severe in such a case.
Any information gained in wiretapping should be considered to be of a higher caliber of reliability than information which is gained through torture or old-fashioned "plea agreements" with criminals. It is undeniably more reliable (being actual rather than virtual) than mere suspicions or suppositions, although that does not negate the value of the latter in obtaining the pure and true justice which is the object of the government at all times.
In summary, the use of the term "warrantless wiretapping" is a red flag, indicating strongly that the person using the phrase is an enemy of the state and is undoubtedly hiding something of vast importance.