In military terms it's highly inefficient to make special units to assault a planet that, as we see in the films, basically anyone can and does survive on... It'd be like making specialized units now to go into Iraq or Afghanistan... We don't have desert combat units because it's not essential, just as we don't have true "arctic assault" troopers like Snowtroopers but we have mountain troops as a special unit... In Star Wars terms it's more believable that snow planets would need specialists over just a hot planet.
The reasoning on the packs is that they're modular clearly in the films... They're not all the same. It's reasonable then to assume that the frame can be fitted with gear specific for any situation. A desert trooper configuration may consist of cooling units, water/hydration or whatnot. A jungle/swamp unit may consist of something to adjust for the comforts of the trooper in a humid situation. Or if, for instance, it's winter on a planet with varying seasons there may be heating units and such fitted to the pack... The Snowtrooper gear may then just be the uniform of a trooper needing to drop into a deep, inhospitable world like Hoth... It's also my belief that "snowtroopers" don't exist either though, as like I said it makes more sense that they change clothes over the Imperial fleets just happening to have all these large units of specialized soldiers readily available for assault at a moment's notice.
In EU the pauldrons and packs are worn by soldiers on a number of planets though, not just deserts, and I have a hard time buying a need for desert specialists, though any troop stationed there on a permanent or long-term basis would of course have a greater adaptation to the environment, but deserts aren't wildly different environments like, say, an underwater stormtrooper may have to deal with. That would be a trooper needing specialist training atop the standard routine.
Sorry to take this a little off-topic, just covering some EU theoretical ground.
