Starting with dangerous: Obviously, if your partner pushes you to hard, you could easily over stretch and tear something. Just like if you try to stretch to much, or to cold by your self and over do it.
Now on to the good stuff: It is absolutely beneficial. The one technique that I use quite often is to have yourself stretch to as far as it's comfortable, have your partner hold it, and you resist against your partner for 8-12 seconds. Then relax and your partner will push the stretch just a little further (as far as comfortable). This style of stretching is great for increasing flexibility and is hard to do solo.
By favorites for this are:
- Place your back against the wall, lifting a leg straight in front, with the resistance pushing down.
- Sideways to the wall, leg straight up to the side, with the resistance pushing down again.
- The butterfly stretch (sitting, bottoms of feet together, partner will be behind you, pushing your knees down).
You could also adapt these to some shoulder stretches. I don't do a lot of those, because I have fairly messed up shoulders, and the joints don't like that kind of stress.
Also note that it is important for the partners to communicate well with each other and for there to be a level of trust. They need to work together and push each other but not overdo it.
Some More information about the benefits of using a partner:
Some of these you could, in theory do with out a partner, but you'll get a better stretch, and be able to push against the resistance better, if it's being provided by a partner. The standing, leg sideways one is a good example, sure, you could get almost the same stretch from doing the splits, but how can you resist against that? For the "butterfly" stretch, having to use your own elbows or hands to push your knees down will take away from your proper posture for the stretch.
Basically, by using a partner for the resistance, you allow for better range of motion and form and will have more steady source of resistance then you could provide yourself.
here are a couple of links about isomentric or pnf stretching: