For me, it's for a number of reasons. First I like the way it looks. The handle is very secure, the blade shape is lovely, and overall it doesn't look like a weapon, it looks like a high quality, classy tool. I like the lock, somehow it feels better on the 710 than on any other Benchmade I had go though my hands. I have a 940 Osborne, same l lock, same general handle shape, but it's not the same. The blade is surprisingly long, nowadays you usually get a handle at least 1/4" longer than the blade, and about 1/2" longer than the cutting edge, while 710 has those almost in balance.
So, when a root in the garden needs to be cut, a serrated Spyderco Millie comes out to play. When fish gets cleaned, Spyderco Catcherman goes to work. But when I'm heading out, it's very likely it'll be the 710 that gets to come along. What it's lacking in raw reativity of Spyderco it makes up in packing a lot of punch in a small, classy package.
By comparison, a Spyderco Military, which I also love and have at least two of at any time, packs the same blade length in a much longer and wider handle. It's a flat ground strong slicer blade and M I L I T A R Y on the blade plus the opening hole bothers some people, and of course if you hapeen to carry a serrated one, heads turn, eyes bulge. But 710 looks simple, streamlined, functional but not too flashy. Butterfly on one side, McHenry-Williams in a non-militarized font on the other side. It looks like a tool, and packs a surprisingly long cutting edge in a short and narrow handle. It's what people expect the knife to look like, a somewhat curving handle of the Military makes them wonder, but straight yet ergonomic 710 does not, maybe because the blade is so narrow, and the Millie blade is so wide by comparison, the "you don't need that much" syndrome kicks in.
Anyways, it's just how it works for me. I only have one 710, the plain edge ones started to get scarce when they were discontinued, but working on getting a second so I can carry without worry. Were they ever discontinued, or was that a figment of my imagination? I could have sworn they were, but Benchmade website has them in stock or back-ordered. Hmm...
Anyways, I can't quite explain it, but I like it.