Okay, so sure, the shields aren't up unless there's a reason, if that was the meaning. They've have been known to go up automatically when another ship enters their vicinity.
But then the shields go down, unless there's a specific reason to keep them up. With Picard Trek, it's often difficult to tell because Picard skilfully avoids prolonged fights, but Kirk Trek prominently features the ship in combat with lowered shields in, say, "Balance of Terror":
- When heading into danger, approaching a ship that has just made a series of surprise attacks, Kirk orders weapons armed but not shields raised
- When the plasma weapon is fired at them, Kirk still doesn't order shields raised, a sage move considering shields did no good for the outposts
- When the Romulans use the nuclear mine, shields aren't mentioned as a factor
- While we could attribute all that equally well to Kirk
constantly keeping shields up, there's the period of "silent running" in between, speaking against such a possibility
In "Errand of Mercy", Kirk orders full shields (or deflector screens) on approach to Organia yet effortlessly beams down moments later. In "Arena", Kirk repeatedly has to tell Sulu to keep shields up during combat. And so forth. Specifically, transporting takes priority over shielding in many a plot.
The cloaking device being:
1. Relatively easy to find on the Romulan ship
2. Relatively easy to disconnect from the Romulan ship
3. Small and light enough for Kirk to pick up and carry
4. Able to be patched into the Enterprise's deflector/shield system by Scotty in a relatively short period of time, especially given that he had no time to prepare because he was out of the loop for most of the mission, and
5. Actually working on Enterprise when they hit the ON switch
stretches my suspension of disbelief to the breaking point.
We could argue that Starfleet only ever launched the mission because they had inside information about all these facts. Keeping Scotty out of the loop was the truly daft part, but I guess the writers wanted Doohan's excellent "baffled and distressed" acting added to the soup.
Timo Saloniemi