First an SSD will be about 3 times as expensive than a conventional HDD of the same capacity.
It will boot, shutdown and run a MBP faster (for heavy duty applications) than a conventional HDD.
It will run a bit cooler and use less electrical resources than a HDD.
If you are a casual user, it is not essential, but becomes a bit of a luxury.
If you use a lot of resource intensive applications, especially in a vocational situation, it may very well pay for itself.
I have SSDs in 3 of my newest MBPs and have enjoyed having them, despite the cost.
If the bank account is lean, consider a hybrid drive that has some flash storage combined with a traditional HDD. They are generally referred to a SSHD's. The price is a slight premium over a conventional HDD.
Ciao