If you can't guarantee you'll have time for all regular meals then MRPs are a good idea to ensure you get adequate protein and other nutrients in your diet. You've made several references to GNC in your post but you'll find much better deals on the Web. Look around at some of the board sponsors like Bulknutrition.com
Also, remember to search before you post, there are a ton of threads relating to MRPs, e.g.
In need of a good MRP
I'm looking for a good (inexpensive) MRP
What is the best MRP out there?