I still claim the exact opposite: items are forbidden from leaving the holodeck in the same sense that water is forbidden from leaving the pool. The point of the pool is to keep the water in. The natural tendency of the water would be to splash out when the pool is in use, but the builders take precautions against this. Similarly, the builders of the holodeck take precautions against the matter leaving the playground.
But those precautions are simple software rules, because whenever the software goes asleep (malfuctions, is commandeered, doesn't care), matter does leave the holodeck, with nary a comment from our heroes. That wouldn't be possible if the no-leave rule stemmed from hardware shortcomings.
So there's a splashguard at the door, usually. But not always. When it works, we get the vanishing book of "Elementary, Dear Data". When it doesn't work (whether by accident or by design), we get the gradual dissipation of complex partially replicated, partially forcefieldish structures as in "The Big Goodbye", or permanence in case of simple replicated substances as in "Encounter at Farpoint", "Angel One" or the piece of paper in "Elementary, Dear Data".
That spectrum of evidence can't be explained if the limitation is all hardware. And IMHO it can't be dismissed, either - not when there's so much of it, and sometimes integral to the plot.
Timo Saloniemi