Pistol squatting is even less than the equivalent of a barbell back squat.
You're not lifting the leg that's doing the squatting any more than you would with a barbell back squat. According to this website
http://www.exrx.net/Kinesiology/Segments.html
a well proportioned leg will weight about 17% of your bodyweight. So that means that for a 200 lb guy, he's doing something like a 166 lb back squat, and that's without all the benefits of loading your back with external weight. So it's even less effective than an 83% bodyweight back squat.
The real benefits of pistol squatting is that they're the only non-equipment way to exercise your legs. The only way to get any gains with them is to do them for mass reps (we're talking sets of 10-20 for 5 sets minimum) or to do them explosively, with a leap for height and distance. That's exactly the way gymnasts do them.
The one legged squat is not hard to work up to, either. Back when I lived overseas for 6 months, I worked up to an easy set of 5 in my apartment in 2 months of fairly dedicated work. I started with the ability to do something like 30 bodyweight squats a set (I'm a smoker so my conditioning is and always has been horseshit).
Step 1
Work up to at least 50 bodyweight squats, if not 100 consecutive. Look up form for these, since they're lightweight you might as well use picture perfect form to get the most benefit out of these.
Step 2
Open a door so that you can hold onto the doorknob or the edges of the door for assistance, lowering yourself down on one leg. Work up to 5-10 easy reps.
Step 3
Lower yourself on one leg, and raise yourself on two. This will teach you the balance of it, and teach your one leg to work on it's own in the eccentric portion. I did these alternating on each leg for sets of 10. If you're still shaky "in the hole", I recommend doing a pause at the bottom for a count of 5-10 seconds. That would be the picture furthest to the right.
Step 4
When you're comfortable with step 3, attempt a pistol. You should have enough strength to perform at least one. For bodyweight exercises I tend to make a rep goal (let's say it's 20 per leg) and I time myself how long it takes to do them. The next time, I try to beat that number. Once 20 gets too easy, increase the reps, attempt explosive reps for height and/or distance, or add weight with whatever weight you have.