Depends on what you are trying to achieve. If you are running out of CPU power on one node, then instantiating a session on the same node probably won't help a lot (but you might still see improvements on multi-CPU machines, as multiple sessions will probable be able to make use of all of them in parallel).
That doesn't mean that loading multiple sessions on the same node isn't useful, you could use it to divide the processing in multiple, independent units (for example, a common approach we see is one session per customer if you need to make sure requests from different customers run independently from each other) and it makes scaling out later easier as well (you can move some sessions to a new node).
But in this case I was refering to instantiating sessions on new nodes to increase the total processing power, yes.
Kris