The anger, frustration and agony behind your query is quite logical and understandable although it does not appear to be the ideal way of addressing the society of lawmen as a whole.If there has been a mechanism or system available anywhere in the world that can judge flawlessly and without trial as to who is guilty and as to who is not, and deliver justice expressly and perfectly like expresso coffee like a vending machine,you would not have queried so.
Before the advent of the system of fair trials, probably there would have been no advocacy for the accused and once someone was accused, punishment would have been pronounced unilaterally, without a trial.Then too there was the probability of injustice being delivered, because of favouritism or vengeance or partiality or bad reasoning or imperfect logic.
Only with the idea of improving the quality of justice administration, fair trials were introduced.The foremost disturbing factor is not the issue of advocating for a person popularly known or publicised to be guilty, but the fact that justice is denied or delayed even after such fair trials.
As long as justice prevails no one would get frustrated or aggrieved. But it does not prevail all the time.That is what is painful. You would not have got frustrated if the 70 year old widow was not deprived of her property. You were disturbed only when she was delivered injustice. Therefore all boils down to justice, as to whether it is delivered or not.
Wikipedia defines:The presumption of innocence (the principle that one is considered innocent until proven guilty) is a legal right of the accused in a criminal trial, recognised in many nations. The burden of proof is thus on the prosecution, which has to collect and present enough compelling evidence to convince the trier of fact, who is restrained and ordered by law to consider only actual evidence and testimony that is legally admissible, and in most cases lawfully obtained, that the accused is guilty beyond a reasonable doubt. In case of remaining doubts, the accused is to be acquitted. This presumption is seen to stem from the Latin legal principle that ei incumbit probatio qui dicit, non qui negat (the burden of proof rests on who asserts, not on who denies).
So, every one is presumed innocent until and unless proved otherwise. That creates the need for a fair trial which creates the need for advocating both parties of the case. What should be desired is timely justice at the end of the trial. That is what is eluding quite frequently. That is what the system is lacking and that is what is not desirable.
As sri.Assumi pointed out, Sir William Blackstone's ratio which is acknowledged worldwide, as a principle behind jusicial systems in civilized societies, calls fpr a fair trial. It sounds absolutely fair and inevitable in order to bring out the truth. One will understand perfectly the need for a trial of both parties to a case, when such person, while innocent gets accused of a crime or fault.
I guess there should be a number of lawmen who would not advocate for Kasab even if the entire richess of the world is given to them as their fees. But not every case is so clear about the guilt of the accused.
I, as a non lawman, would request lawmen not to get offended by the query, considering the agony behind the query, which I presume has been caused by injustice prevailing in the society.