To say that some types of crime should be excluded from plea bargains is to imply that plea bargains are some sort of a deal given as a gift to defendants. They are not. Instead, they are given at the discretion of the prosecutors and judges, largely for the benefit of the prosecution.
The prosecution is going to charge people with the most serious crime it thinks it can win conviction on. If it offers a plea bargain, it is because there is some doubt about winning the case on its merits. Prosecutors are not going to say "I can prove 1st degree murder but I'm just going to let you get away with 2nd because I'm nice." If that were the case, then maybe we should exclude some kinds of crimes.
But it's not -- the prosecution will be as harsh as it can be. So I don't think plea bargains are being given away too easily so as to help defendants.
First of all, I take issue with your characteristic of plea bargains as "controversial." Every defendant is entitled to a trial by a jury of his peers, regardless of the strength of the evidence confronting him. At the same time, our court systems are overloaded. Many times, a defendant is allowed a reduced sentence for a guilty plea as an acknowledgement that he has assumed responsibility for his crime, and has not put the system to the burden of proving the obvious.
There are some crimes so heinous that prosecutors believe the only just punishment is the maximum punishment. If one has committed a particularly brutal murder, or other crime which might justify a sentence of death (in those jurisdictions in which the death penalty is still practiced), the prosecutor may feel that anything less than the maximum is inappropriate. In such a case, the defendant can still plead guilty, but will receive the maximum sentence.
If a prosecutor feels that there is the possibility of an acquittal, he may decide to cut his losses and accept a plea. This is one of the more unsavory elements of plea bargaining; but it is preferable to seeing an obviously guilty person acquitted.
A final note: In most jurisdictions, plea bargains are not binding on the trial judge. Most judges are strongly inclined to accept the recommendations of the prosecutor; but are not obligated to do so. If the Judge does not agree to the terms of the bargain, then a plea of not guilty is normally entered.