When you push out a pin, it's not easy to push it back in so that the link doesn't bind. And if it binds, it tends to wrap around the small gears in the derailleur and cause the chain to slip. When this happens on a BBS motor, you really feel it. If you fiddle with the tool and do it by feel, you might get the link to work freely. Not easy out in the field or even at home.
The bikes with 8-10 speeds use thinner chains where I guess it's even harder to push a pin in correctly.
When I asked about this in the bike mechanics forum, they told me to use master links. KMC, the chain maker, also tells you to use master links. Anyway, they're not expensive and work nice, except you might need a link pliers, and you still need a chain breaker too.