The alignment tool is helpful to have, because accurate hanger alignment is the foundation of good shifting. An enthusiast who often works on bikes would enjoy having the tool. A professional should certainly have one. You have one bike and it's been seven years, so you could either buy the tool or not depending upon your preference.
I guess I would say to buy the tool if you enjoy troubleshooting bicycle problems as an activity in itself. If yo ulove to work on bikes, you'll enjoy having the tools. Otherwise, you may find it easier to pay a shop to check the alignment.