It might even seem fundamentally right to you; instead of changing the complete set of 4engine-mounts you go ahead and change just two and in some very weird cases, just one ;also in the case shock absorbers, typical car owners will rather swap the two worst affected shock-absorbers and leave the rest to be replaced on a later date. Some might proudly affirm that if it isn’t broke don’t fix it; but as a matter of fact, based on empirical findings and recommendations by car manufacturers, mechanical(mostly) car parts subjected to high stress, pressure and heat and that can equally wear and tear should be replaced in sets. The common reasons that make carrying out replacements in this manner are hinged on the need for absolute safety while driving, continually achieving uncompromising performance and balance.

Below are some pointers that elaborate on how necessary it is to painstakingly replace your crucial mechanical/some electrical spare-parts in sets where applicable.

Mechanics might be encouraging you to save: But the reality is that they encourage you to spend less at that time to guarantee a speedy revisit, because they are concretely sure that if you go with their suggestion, you will be right back in their workshop in no time. The outcome of this is spending more on labour, logistics and time.

Work load is transferred: Some car parts that require being swapped in sets are bearings, shock-absorbers, spark plugs, engine mounts, tyres, CV joints (constant velocity joints), etc. In each of these cases, whenever there are improperly functioning members in a set, the work load is automatically transferred. What happens when you don’t change all the members of the set (which were installed at the same time) is that the newly replaced member(s) bear all these extra carried over loads (forces in most cases) transferred by the retained members of the old set. It’s a very simple see-saw kind of situation where both parties are partly responsible for the positioning and directional movement of the mechanism.

You risk higher fuel consumption: In the case of spark-plugs, tyres, shock absorbers and engine/transmission mounts, this completely stands true. The all important aim of saving money is defeated when you choose to retain members of an old set of car spare-parts.
source:http://www.autobaseafrica.com/swapping-in-sets-vs-swapping-singly/