It's reasonable only if they were expecting the PCs to imminently break the door down. Obviously you can't maintain combat actions like Ready indefinitely, just as you can't (IMO) pump out at-will close blast attacks for hours at a time! If you're using them you are effectively 'in combat', with all that implies in terms of hyper-alertness, fatigue etc.
Movie example: The Corellian Corvette marines/Senatorial Guard at the start of Star Wars had a Readied action to shoot the first Imperial Stormtroopers through the Corvette airlock. They were effectively 'in combat' at that point despite there being no stormtroopers to shoot at. They could maintain that for a few minutes ok. It would also be reasonable to let the stormtroopers have a total defence action + move as they came through the airlock, rather than a move + fire, if they wanted. That would work for them since the Readied Senatorial Guard would get to Interrupt the stormtroopers' move with their standard action (fire), getting to fire before the troopers did.
Normally though the
surprise round rules work fine and give more flexibility to the ambusher; I'd only use readied actions in the rare situation where both sides are aware combat is about to start, initially neither has line of effect on the other, and rolling init to see who goes first seems inappropriate. In the stormtrooper vs Corvette guards example it would probably all crunch back down to mutual readied actions (both sides being Readied to fire when the airlock door blows) & I'd end up just rolling init to see who went first.