All of the above.
Safety - If someone is doing a good job, I'll fight like crazy. That has resulted in injury to lers in the past...fortunately minor. Also, because of my history and the possibility of flashbacks during more intense play, it's best for all involved if I'm restrained. So, if we're going to do more than a bit of goofing around, it's safest for me to be restrained.
Enhancement - If I can't go anywhere, we can keep going longer. And that's a good thing!

If I'm playing with a ler who knows me well and whom I trust, I like to have my limits pushed. If I can get away, I will try to do so once it gets intense. If I can't, I'm able to stop fighting so much and just enjoy it and let it take me away to lala land.
Helplessness - As many know, I can be a total control freak...a result of spending most of my life in jobs and situations where I was the only one around able/willing to take care of things. It's just automatic for me. But, when it comes to tickling, I enjoy giving up control and being totally at the mercy of the ler...provided that they are safety conscious and aware of things that play into this for me so they don't get any nasty surprises.
Other - Fighting less, as a result of knowing that it will do me no good, also allows for longer play time because I don't wear myself out with the fighting as much. and, as I said above, that's a good thing!
