Here's the problem: orbital debris.  For example, before Challenger was destroyed, one of it's windows was hit by a paint fleck the size of a postage stamp that left a pit a quarter inch deep in the glass - it was like getting hit by a bowling ball going 60 MPH.  That was by no means unique - NASA has had to replace just the windows on the Shuttle over 50 times.  As a result, the Space Station has a HUGE mass investment in debris impact shielding.  Talk about gold plating - it's not actually MADE of gold, but by the time you figure in the launch costs, it might as well be...

So inflatables have two big gotchas.  Sure, the astronauts want high-school-gym size rooms to soar around in that inflatables could easily give them.  Unfortunately, a big inflatable room is a MUCH bigger target to get hit by space debris than a nice, cramped compact module.  We just can't afford to launch the shielding mass required to protect them from punctures.  

Also, the mass required by the shielding for an inflatable is just the beginning.  Then you gotta fill it with air.  Air is heavy.  Air has to be lifted from the surface of the Earth at $10,000 per pound.  Elbow room and soaring overhead expances for astronauts cost A LOT in air mass.  By current NASA rules, whatever the air requirement is, double it - you have to prepare for a puncture-and-evacuate contingency with complete decompression before repair begins.  Thus you have to have a complete change of air, stored in high pressure tanks that are themselves a very heavy and dangerous subsystem in space.  You have no idea how many hours I've wasted sitting in meetings arguing over the relative safety merits of various emergency repressurization systems - NASA will not leave home without them, but they sure don't like them, and for good reasons.  If one of those high pressure tanks ever gets holed by space debris...well, the sudden uncontrolled disintegration of a pressure tank is the definition of a fragmentation bomb.  

I also think inevitable catastrophic debris strikes are the one thing that will make an otherwise promising space elevator idea ultimately infeasible.  However, I still think we'll one day see inflatable use on interplanetary trips when we can get out of the flying garbage dump we've made low Earth orbit.

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