Stealth isn't "undetectability" even in Trek terms. The "practical invisibility screen" from balance of terror still allowed the cloaked vessel to be tracked, though not targeted.
Or think of submarines. They are, undeniably, the paragon of military stealth; they are also detectable by some fairly simple sensor devices like sonar and magnetic anomaly detectors. Even so, submarines remain stealthy enough that they are not easily noticed and can therefore avoid casual searches until it's too late.
The same must be said for Trek sensors, the often overlooked fact that the effective range of any sensor device depends ENTIRELY on what those sensors are looking for and their actual resolution at those ranges. A sensor sweep for something the size of a starship could take a few seconds or several hours depending on the resolution of your sensors and the range of the sweep. A cloaking device would have the effect of either 1) increasing the amount of time it will take to search a given area or 2) decreasing the maximum range within which a sensor could locate the vessel in a reasonable amount of time.
It's the difference between going into combat wearing camouflage and going into combat wearing a bright pink jumpsuit with hologram lamps on the shoulder.
No, even the "practical invisibility screen" wouldn't work. You might be able to bend visible light around a ship, but that wouldn't be enough. If you're not dumping lots and lots of waste heat which is visible on infrared, your ship will fry like an egg. All a searching ship would have to do is switch over to the infrared spectrum and the cloaked ship should stand out just as well if not better than by visible light.
There is no way to camouflage yourself. A starship will stand out as if it has spotlights on its shoulders no matter what it does. Check out this link, if you have the time. Its a really interesting read:
http://www.projectrho.com/rocket/rocket3w.html#nostealth