Actually Tatran, I don't have the spreadsheet in front of me, but there are much more than just 3 AIs that can plot at please - I say close to half of them can, but have never added them up. Even Mansa can plot at pleased. Cathy is the only AI that can be bribed at Friendly.
I rarely use Defensive Pacts. Only reason I do so is if Permanent Alliance is turned in special forum game, or if I'm going a small peaceful route and want a DP with a large power AI which is very rare. It can be good in OCC to keep you safe, but in the right circumstances...you don't want to DP with someone that is just going to drag your tiny empire into wars you don't want.
Not sure how much of a deterrent a DP is, especially if someone is already plotting on you or your partner, but I suspect that factors in toward someone actually starting to plot.
The problem with DPs is you do run the risk if getting called into a war that you don't want. It's fine if you are prepared for it. But it could be the partner that someone is plotting against or planning to plot against. If you are already a strong warmonger ready to take of the world anyway with little resistance than DPs are worthless, and they get canceled the moment you DOW anyone.