even with all these terrorist attacks lately the chance you will actually encounter one is very slim. But if you do encounter one, a vest would be very welcoming ! More likely things are robbery, kidnapping and stuff. A vest can safe your life in that situation. But even the most modern vests are certainly not as comfortable as just wearing a t-shirt. They are hot, sweaty, stiff, heavy and a good one cost a fortune. Wheather to take a vest or not is all personal. Do you thing the chance you will encountering a situation wenn a vest could save your life high enough to pay for the vest and actually wearing it everyday? Thats the answer you should ask your self.

btw a level I vest is beter than nothing, but it's really gives very little protection. Take atleast a IIA vest or higher.