The downside is that there is probably some device or energy cost for keeping that Pocket Dimension open / live-able.
Nothing is free in life, there is always some sort of energy cost.
Keeping a Pocket Dimension of some form open probably doesn't come for free and there is a significant / on-going energy cost that isn't trivial to have your own pocket dimension.
If we follow inflationary theory then a pocket or bubble universe could be stable depending on its initial metric conditions, so while there'd be a high (VERY high) initial energy cost the bubble universe wouldn't need a power source to maintain it – it would just sort of be there. This is analogous to the "warp bubble" of Miguel Alcubierre's warp metric – once the bubble is created it's a "real" object, and one of the big unsolved issues with Alcubierre's original theory and the subsequent revisions are that it's not clear how to shut it down or open it up, and thus how to get anything out of the bubble's interior.
Of course if you were going to build a ship in that bubble universe you'd need power to run the ship, but that's a different issue. (Never mind how you can get into and out of a causally disconnected part of spacetime – a door would be VERY interesting from a physics perspective, and is potentially a mathematical impossibility). But the point is that while the TARDIS running out of power might kill the lights and life support, it wouldn't necessarily make the interior space collapse.