In paying attention to my own browsing habits, I would assert that the sub-title, even though not searchable, gives browsers more "incentive" to view the listing, instead of scanning over it in the gallery view. Though I don't imagine the .50 fee, would make it worthwhile on low profit margin items.
A fair example of this was in looking for poly bubble mailers, which I purchased from "Total Shipping Solutions" This seller offers combo packs of these mailers, which is a boon to a small book seller such as myself, who has books in a wide range of sizes, but does not yet have the capital (or storage space for that matter) to invest in cases of different size mailers.
The prices are reasonable, AND the seller also offers combined shipping at no extra charge for additional items. This got my attention, and thus my business.
In the sellers subtitle descriptions, are listed the "quantity and sizes" which are included in the combo pack. (At least on some of the listings) That would be helpful to the shopper, so they can see right away, how many envelopes of each size are included. Particularly in searchable auction listings, when the items would be displayed among a sea of similar items.
Shipping envelopes are not exactly a "fun pastime" to be browsing for on eBay. They are purely functional items, and shoppers will be looking for the BEST VALUE for their dollar. This seller recognizes this, and has come up with, what I see, as a good idea to stand out from the crowd. The seller may not offer the very lowest price for the mailers, but offering a mixed lot of sizes, makes the little bit extra, worth it.
LDCdesigns
http://shops.half.ebay.com/ldcdesigns