The global smartphone market — which somehow posts continued growth even as the world remains in a general economic depression — is said to be worth more than $85 billion. By 2015, that number is expected to eclipse $260 billion.

While it isn’t clear whether the accessories industry figures into or alongside these numbers, it is clear that that mobile add-ons are selling faster than ever. According to a mid-2010 HEXUS.net report, market analyst ABI Research (who requires a costly account to view specific figures) has concluded the following:

It seems that aftermarket accessories – those not included with the original purchase – produced worldwide revenue of $26.5 billion last year, a figure that is expected to double by 2015.

We all knew the accessory game was a substantial player (most of us own several accessories, after all), but tens of billions of dollars annually? That’s pretty staggering. It’s almost impossible to believe, really, until you start thinking about the old American concept of “planned obsolescence.”

Popularized by industrial designer Brooks Stevens in the mid-1950s, the phrase (and practice) became most obviously commonplace in the US automotive industry of that era. Defined as “instilling in the buyer the desire to own something a little newer, a little better, a little sooner than is necessary,” planned obsolescence is practiced the world over to this day.

In the technological sector, planned obsolescence is something of a misnomer. Because of the rapid pace of hardware development for computers and other consumer electronic devices, the aspect’s existence is more innate than intentional. For tablets and mobile handsets specifically, rapid, radical hardware changes are inevitable. Still, many companies unnecessarily tweak the physical characteristics of their various devices’ successors in small part to promote the purchase of new accessories. In this way, the entire mobile and mobile accessory markets straddle the “planned obsolescence” line.

There will be a time when device components are so small that the limiting factor of physical construction will be the user’s own interactive comfort. Consider how much thinner the iPad is likely to get, and it’s easy to see that, soon, Apple’s tablet will be little more than a comfortably-beveled slab of glass. Over time, though, that physical design will still be changed and modified because Apple knows — as do all competing mobile brands and manufacturers — that to let a style stagnate is to extinguish the product’s healthy accessory segment.

It is for this reason that I’ve only owned three mobile accessories these last several years. The first — an HTC-branded leather cellphone pouch I found under an ex-roommates forgotten laundry — I’ve used with my last half-dozen handsets. The second is my nifty little oStylus, and the third is a blue iPad 2 Smart Cover. I plan on getting a universal GPS mount sometime in the future.

But I’ve always wanted more. For my original iPad, I had my heart set on a gorgeous Vaja case; but, at over $200, I wasn’t willing to shell out the cash without the assurance it would fit next year’s model. I guessed it wouldn’t, and I’m glad I did. (In fact, my gal’s leather Vaja 3GS case is the only reason she didn’t upgrade to iPhone 4 when it came out last year.)

For my iPhone 4, I almost pulled the trigger on several supporting items. There’s the JOBY GorillaMobile, the Exovault line, the Vapor Pro LE, this crazy thing, and mophie’s battery-extending juice pack air. Shoot, I’d even grab one of those upcoming Kogeto Dots! But it all boils down to the same thing: They’re only useful for a very limited time. Thus, the sole protection my iPhone has is its promotional black Bumper, and I only use that if I’m out in the elements.

Because the return on investment for accessories is so low to me personally, and because of the inherent uncertainty of future compatibility, I’ve learned to resist the constant urge to purchase new stuff for my new stuff.

As mobile devices get more and more powerful while growing (shrinking?) thinner and thinner, it would be nice to see a concerted effort by the industry’s manufacturers to work towards single, rarely-changing form factors for flagship products. If the iPhone 4S/5 retains the same physical dimensions as it’s forebear, it will establish a happy trend in Apple’s recent handset history, as — for the most part — iPhone 3G add-ons were compatible with with the newer 3GS, which meant extended utility for expensive accessories. If they can stick to that two-year external design cycle going forward, Apple might change my mind down the road. Until then, though, I just can’t spend much money on embellishments that functionally expire.

I understand this all might sound silly in light of the fact that I’m perfectly willing to upgrade my phone each year (even though it costs quite a bit more than purchasing the requisite accessories), but I’d like to see mobile manufacturers take another lesson from the auto industry: Internally tweak product lineups yearly, but give us a chance to get the most from our preferred extras. When I get a new truck, for example, I want my old tonneau to fit. When I get different tires, I’d like to use the same wheels.

Drastic yearly changes can sap enthusiasm as quickly as perpetual software fragmentation. I think a two-year cycle would be a welcome standard for stylistic alterations, and I really hope Apple mostly keeps with it. I can honestly say I’d be thrilled if the iPad 3 mirrors the size and shape of this slate in my hands and the next iPhone looks like the one in my pocket.

I’m a stylin’ guy (don’t you ruin my fantasia!) but not a particularly wealthy one, so it’d be nice to get a little more mileage out of these cases and dongles and docks and mounts and what have you.

Although, truth be told, if Case-mate released an iPhone 4 or iPad 2 Recession Case, I’d probably spring for the 10-pack.