5) Convinience and Not Caring
If you buy 5 keys a month, you would save about 6 USD when trading. For that you have to find some dude who trades, and then give him personal information. That saves you roughly the equivalent of 2-3 Tall Latte coffees at Starbucks. For the risk of losing a lot of money because of violation of TOS/EULA.
It's okay to have more than one engi on offensive, if it buffs up players or corners red. But it drove me crazy that he changed class a few moments after I did, when it was obvious that he was the only engi left.