Why in the world would you want 100k of credit?
It increases your score because you have a lower credit utilization and you have more available to you (supposedly signaling how responsible you are).
I just have a lot of cards. If they want to give me bonuses for signing up I will take them. Free money.
Plus usually you can call and get the second year free too.
Regardless, I can cancel w.e. and the increase in my credit score is worth it. At the moment if I want to buy a house (for example) I get the best rates on loans