I can see the issues, yes, but it's not much different from being able to summon elementals or take them as familiars.
My real opinion is that the druid moved from OMGWTF combat character to secondary combat character who has a lot of versatility & utility going on. Have you looked at what a wildshaped druid is doing in combat these days? They're roughly on par with a bard.
Also, I think this might be an overreaction. While it may not be at-will, diviners have always had access to arcane eye and clairvoyance, and the ever-lovely detect thoughts.
You can combat this by using stone floors, facing your players off with burrowing monsters, or having anti-magic fields set up in some rooms. In 3.5, anti-magic suppressed supernatural abilities, though I haven't read up on it in 3.75. I have to imagine that the first time your druid finds herself in human form 10 feet below ground she'll think twice before attempting a plot-bypass.
It's not as extreme a step to that ability as fly was. You can get fly as a 5th level wizard. That single step requires a designer to completely redesign encounters to challenge the party that can go around obstacles by using the third dimension.
That said, it doesn't account for springs in the rock (that would block or even injure the elemental), magics to protect key areas, and other weirdness. If the party gets a map of the dungeon, well cool. Now they know what to look for. Might make them more confident and prone to heading into areas with problems. Just because the druid can scout ahead those doesn't mean perfect, eidetic recall of the exact location. Good way to replicate this: show the player a map for about three minutes. Let them study it even. Then take it away and hand them a blank sheet of paper to sketch it out on again. They'll get stuff wrong. Then you get to have your fun.
Don't forget you can't see anything while in the wall, so you would have to go with your head above ground so to speak and that would draw attention... not something that is healthy for a scout.
Another solution is that you obey movement and placement rules, which generally means that the druid's body has to exist in 5' increments. A small elemental then either has to be inside the wall/floor/ceiling or outside of it. Nowhere in-between. Same with medium elementals.
They are likely less inclined to scout ahead if they know the enemy has anti-earth elemental protection ahead.