Although I think the topspin lob is safer, more offensive, and preferable in most cases, the slice lob has it's place.
I use slice lob when I get pulled wide on BH side like WW Volley as well. It's a purely defensive shot to force the opponent to hit an extra ball to finish you off. It takes very little time and movement to pull off. Assuming you step across your body to reach for it, as you extend your arm to the ball, that action automatically puts backspin on the ball because of the open face of the racquet cutting past the bottom of the ball. I use SW forhand grip and if you try to hit the ball on the backhand side with that (or other closed forehand) grip, you'll notice the racquet is already quite open, so I dont need to change grips when going for a backhand slice lob. In conclusion, basically all you have to do to hit a BH slice lob is to stick your racquet out at the ball with a firm wrist, as opposed to worrying about taking your racquet back and getting a good swing up on the ball for the topspin lob. Sure it's harder than it sounds and it'll take practice to get decent with placement, but in the mentioned situation, it's the better option. (Even if you throw up a sitter, if you got good backspin on the ball, it could make the ball pop off the opponent's racquets downward, helping him to hit his easy overhead into the net.)
Sometimes I'll use a forehand slice lob if both the opponent and I get into a volley rally close up at net. Like the backhand slice lob, it takes less body movement and preparation than a topspin lob and it looks deceptively similar to just another routine slice volley... until it innocently slips over their heads and past them. Never have I made a lob volley where the look on the opponents face wasn't priceless.
I play tennis for the creativity, shotmaking, and fun of it, and a slice lob is a nice, but probably not necessary, option to have to keep the exciting rallys last longer.