There are some issues with doing that though, even if you read it through the fermentor (which is tricky even with a glass carboy or better bottle.)
Main thing is that getting an accurate reading is pretty tough for a number of reasons. Krausen is going to make it float at the incorrect level, and even when the krausen falls, it will leave a residue (identical to the krausen ring left on the inside of the fermentor), which will add weight to the hydrometer (which is sensitive by design) and make the gravity appear lower than it actually is. And that's assuming the brown krausen residue doesn't obscure the numbers completely. You can also forget about doing this while dryhopping.
Perhaps the biggest problem though (because it's hard to detect) is CO2. CO2 being produced can raise the hydrometer higher than it should be. Even when using the hydrometer on a sample, it's considered important and good practice to give the hydrometer a little spin to get the bubbles off the bottom of it, and that's after only being in the beer a few seconds! In the fermentor, bubbles are bound to collect on the bottom of it and raise it higher than it should be, giving an artificially high gravity reading.
Also, during an even SLIGHTLY active fermentation, the natural convective movement of the beer in the fermenter will also push it upwards.
Mainly though, I just don't see the point. After my first few batches, I stopped constantly checking the gravity - it's unnecessary, and exposes the beer to further oxygen and contamination. In fact, every experienced brewer I know personally is the same way. Unless you are a commercial brewer whose bottom line depends on high turnover (and have fermentors capable of drawing off samples in a very sanitary and even oxygen-free way), it's usually best to let a beer stay in the fermentor 3 or 4 weeks, and you KNOW a typical beer is going to be done by then. Then, when it gets bottled, kegged, or racked to another vessel, it's a great opportunity to also draw off a sample to check the final gravity, in order to make sure there weren't any serious underattenuation problems (very rare when you're patient) or overattenuation problems (sign of an infection), as well as to just keep track of the final gravity and determine your ABV.
Basically, I check my gravity 3 times - pre-boil, OG, and FG. In my experience, it's pointless and even detrimental to mess with your beer otherwise - which, for reasons stated above, will be necessary even if you leave a hydrometer (or similar floating instrument) in the fermentor.
