I was playing Jetpack Joyride, and when I was equipping some Gadgets, I was wondering what the use of the Nerd Repellent was. To me, it seems like there is no effect of it in the game except for stopping you from achieving Achievements related to knocking over Scientists...
Sometimes there are challenges that require you to get a certain distance without knocking over any scientists. Using the Nerd Repellent is one way to make those challenges significantly easier.
As pointed out in the comments, there is also a challenge that simply requires you to use the Nerd Repellent.
I find the nerd repellent very useful in the following cases:
- you need the mission called "don't harm any scientists for xxx meters"
- you need to complete the mission called "buy the nerd repellent gadget and use it"
Furthermore, the nerds (scientists) will be replaced by tumbleweed and if you check the statistics, you will see how much tumbleweed you kicked :)
Also, after cashing in your medal for completing the game there is a statistic the records the number of tumbleweeds you have destroyed. Tumbleweeds are only accessible through the Nerd Repellent gadget.
There is a mission rated 2 stars called: Purchase and Equip the Nerd Repellent Gadget. You get it quite soon after getting a badge. I've had it 25 times being on level 366 with 24 badges. Apart from that, it is totally useless because it stops you from killing scientists. I love doing that.