The Rambam (Sanhedrin 4:8) writes that smicha can be given to a judge for a specific purpose, e.g. a judge may be given smicha to decide torts, but not for capital punishment. However, for a judge to receive any form of smicha he must be proficient in every area of halacha. In other words, even if that judge were receiving a smicha "license" only to decide torts, he must be an expert in capital cases and capable of deciding them.
The Minchas Chinuch notes that among the distinctions listed by the Mishna (Sanhedrin 32) between the requirements for courts judging torts vs. the requirements for courts judging capital cases is that a mamzer or ba'al mum may judge a tort case but not a capital punishment case. How does this make sense according to the Rambam? In order to receive smicha to judge even torts, a judge must be capable of receiving smicha and judging every type of case, even capital punishment cases. If a mamzer cannot judge capital punishment cases, he should be incapable of receiving smicha even to judge torts. The M.C. does not resolve this issue.
Why would a judge need to know everything if he is only pakening in one limited area?
The gemara (Kiddush 10b) records the challenge made to one of the Tanaim: "Baki atah b'kol chadrei Torah v'lidrsoh b'kal v'chomer ei atah yodeia?!" - "You are an expert in all areas of Torah and do not know how to darshen a kal v'chomer?!" At first glance this is a hard point to understand -- what does being a baki in shas have to do with drawing a logical inference? R' Elchanan (Koveitz Shiurim, see post here) quotes that R' Chaim Brisker explained that while an inference may be logically compelling when seen in the narrow context of one sugya, it may prove totally erroneous when measured against the larger background of kol haTorah kulah. There is no such thing as being an expert in dinei mamonos to the exclusion of all else, or paskening issur v'heter without knowing even kodshim sugyos. Truth in Torah can be arrived at only if one has a grasp on the breadth of Torah as a whole.
Mashal l'mah hadavar domeh: A person can zoom in with google maps and look at one isolated few blocks and figure out within those few blocks the correct set of turns to take to get somewhere. This person will be 100% convinced that their "derech" is the correct one. However, someone else who sees the map as a whole may disagree. Looking at those isolated few blocks does not take into account the construction that lies further down the road, the detour that will be required later, the shortcut that comes up later on it you take the other fork in the road.
The reason a person should consult gedolei yisrael on questions of halacha and emunos v'deyos is because even if a person feels that he/she is an expert in a particular little 4 amos of the world of halacha, he/she is looking at no more than a single road on a much larger map. Even to decide between competing opinions and views in a limited area requires knowledge of what Torah as an organic whole means.