should I always get an addon?

[Yoshi63]

If you are a good winning player... Always add-on/rebuy/double-buy (unless you are giving up on the tournament). It is +EV not so much because your ROI% will increase (it will actually remain about the same), but a bigger investment means a bigger return.

If you are a losing player who plays as a hobby... do whatever will provide you with the most entertainment in your own opinion.