The leather cases do look pretty nice. I had one for a bit, but switched to the silicone case after a month and used it for the last 11 months and liked it better....
1. It adds a little extra grip when holding the phone.
2. If you drop the phone, it absorbs more of the shock. I don't really recommend testing it, but the phone in the leather case sort of just falls to the ground. In the silicone case, it bounces off the ground a bit. I've heard in some reviews it absorbs more of the shock. But I don't drop my phone all that much anyways - maybe 3-4 times in a year.
3. It stays in my pocket better than a leather case... in the summer wearing shorts, my phone sometimes slips out when naked or in a leather case. With the silicone case, it adds just a little extra grip to be more likely to stay in my shorts. The silicone case still is easy to take in and out of pocket without gripping fabric, but adds just a bit extra grip to stay put in there!
You can't really go wrong with either. They both hold up well to normal wear and tear. I will say though, that the new 6s silicone cases seem to have a bit smoother feel to them, not as grippy. I bought the new 6S midnight blue case today and it is great... I actually prefer it to the leather in the same color.