Assumption of risk was a traditional defense that took two forms. In its primary form, it denied that the defendant had any duty to take steps to protect the plaintiff. As we have seen, this defense has been totally emasculated by the broad conception of duty courts in Michigan and elsewhere have used. Defendants are now said to have a duty to protect plaintiffs from unknown diseases (Koski), from their own intentional misuse (International Harvester) or from injuries resulting from slamming their car into a bridge abutment at speeds of up to 35 miles per hour (Rutherford).
In its secondary sense, assumption of risk was used to negate a defendant’s breach of duty by arguing that the plaintiff, knowing of the breach of duty and its hazards, voluntarily undertook the risk anyway. In the 1950s, it became fashionable to view this secondary use of the defense as merely a form of contributory negligence,  a view soon adopted by most states.  Michigan followed this line and abolished the defense, save for employer/employee disputes, in the 1965 case of Felgner v. Anderson. 
With contributory negligence an available defense, it may be that abolition of assumption of risk as a separate defense might not have had a negative impact, had "duty" not been broadly interpreted and contributory negligence so narrowly interpreted as to make both concepts meaningless. Even then, however, assumption of risk is probably a better concept. If the plaintiffs in International Harvester and Ross Chemical were not negligent (it appears, at least in Ross Chemical, that the plaintiff had fully considered potential consequences of his actions and acted with care), they were clearly "assuming the risk". If this is not a defense, the manufacturer is attacked from all quarters – he faces a strict liability standard should anything be wrong with his actions or product; even the remotest connection between his product and the injury will be deemed sufficient to establish causation; and he is provided with no defense based on the plaintiff’s behavior. It is a situation that makes liability unpredictable and is rife with moral hazard because the plaintiff is not held accountable for his actions.
Assumption of risk is, ultimately, another expression of the contractual notion that people can agree on who will bear various risks of an accident occurring. Its abolition, therefore, has much the same effect as the destruction of the contractual notion of warranty. It decreases consumer choice, damages efforts to control and predict risk, removes incentives to safe behavior, and encourages morally hazardous behavior by plaintiffs.