Almost always. In order to establish malpractice, it will be necessary to prove what the standard of care in the community for that professional for handling a similar matter is. Lay people do not have the necessary education, experience or skills to act as a professional -- or to gauge what a professional is supposed to do or refrain from doing in a particular situation.
In order to determine what should or should not have been done in your particular circumstances, someone with the requisite education, experience and skill would be needed to establish what the standard of the community is. Many malpractice lawsuits are won/lost based upon the effectiveness of the expert.
Keller and Keller believes that experts should always be incorporated when possible to help solidify your case.