Considering we are speaking generally...
One common reason for using multiple instances, is for improved reliability.
As one server statistically could go down, but multiple down servers is much less probable.
If you are referring to using multiple instances on the same server, some benefits still do exist. There is reasons to run this type of setup, one would be data separation. You also gain some reliability due to potential process crashing issues. However, those same benefits would be better served across multiple servers.
In a completely hypothetical general situation, with absolutely no mitigating factors, the potential benefits don't justify a more complicated situation.
Mitigating factors: an unstable process, lack of concurrency, etc...; would change the question, and possibly lead to a different conclusion.
Specific to memcached, which is considered a stable and threaded process, the benefits don't justify.