Disclaimer: I own my own & develop Yahoo Stores, and have since 1997 before Yahoo! bought Viaweb. I'm also friends with a lot of the folks out in Sunnyvale, so I'm admittedly biased. I love Yahoo! Store.
Yahoo! has finally separated STORE and SHOPPING into two separate entities.
YAHOO STORE is the hosting + shopping cart + ecommerce tools. YAHOO SHOPPING is the shopping engine with traffic from all the Yahoo! properties.
Anyone can sign up for SHOPPING through Yahoo! Product Submit, just like you can for any of the other shopping engines like Shopping.com, Dealtime, Bizrate, mySimon, etc.
OVERTURE listings appear above and to the right of SHOPPING listings, so that's a good way to get that traffic if you don't want to sign up for SHOPPING/PRODUCT SUBMIT.
Yahoo! used to take 3.5% of sales from SHOPPING traffic, but have now switched to PPC so they get paid whether or not you sell something.
In the good ole days, STORES were included for free and they only took a % of sales over $5000 a month. I remember when they also listed 3 Yahoo! Store merchants above the featured merchants based upon the relevance of your Yahoo! Shopping directory listing, which you could edit on the fly. That was fun!
That was when YAHOO! was using the millions of products from thousands of merchants to build their shopping engine for the premium / RMI merchants who paid up to $250,000 to be listed as FEATURED MERCHANTS.
YAHOO STORE customers (a.k.a Merchant Solutions customers) get 20% off PPC traffic from Yahoo! Shopping. The PPC traffic is set at a fixed rate for your industry, based upon what products you sell. Most of my clients either opted out of Yahoo! Shopping when it changed over about a year ago, or get such a small percentage of their sales from SHOPPING, it really doesn't matter.
If you sell jewelry or flowers the PPC rates are high IMHO.
"ANY BENEFIT?" I like Yahoo! Store because of the stability of the platform, ease of use for 'civilians,' and the suite of marketing & traffic analysis tools that come with MANAGER.
YAHOO! STORE is a pretty robust platform, too. I've seen stores with 10,000+ folks a day hitting their sites. MERCHANT SOLUTIONS is their answer to folks upgrading from Yahoo! Store to other more customizable platforms.
I have clients who do millions of dollars per year through their Yahoo Stores, and the stores do well in organic search with minimal SEO. The static html pages are wonderful for search engines.
Yahoo! Store is a little pricey for hosting, but I'm not a propeller-head by any stretch, so I like how they take care of the server stuff, and I can spend my time trying to sell more stuff.
Sticky me if need be.