I had the same doubt and so decided to call the kit manufacturer directly. Their response was -
either way works. However, there are several critical things that new brewers overlook or screw up while re-hydrating the yeast.
The reason the kit instructions indicate "DO NOT REHYDRATE" the yeast is because if this process isn’t done just right it will greatly harm the effectiveness of the yeast and more than likely kill it. The yeast packet instructions make it seem like a really basic process, however a couple crucial key points that the yeast manufacturers leave out on the packet is;
Firstly, you cannot use chlorinated tap as contact with this will immediately kill the yeast.
Secondly, they fail to mention that while stirring the yeast into the water, for rehydration, if done too vigorously it will harm the yeasts effectiveness and ability to reproduce and ferment properly.
Thirdly, they also fail to mention that if the rehydrated yeast is at a greater temperature differential than the wort you’re about to pitch it into, you will thermally shock the yeast, affecting its effectiveness - with the potential of even killing it. All these things can cause great frustration to a novice brewer who’s not familiar with the technical side of the brewing process and can potentially ruin a batch of beer which can be costly depending on the kit and could cause them to not want to brew again.
As you can see, there can be several areas where the novice brewers can mess things up.
Lastly, the yeast packets don't have enough space on the back to include all the above instructions. So they ask the kit manufacturers to include the "DO NOT REHYDRATE" instruction on their kit instructions. Now, if one were to go the yeast manufacturers website and check out the "Techincal Data Sheet" of a particular yeast, then the data sheet would include all of the above instructions and much more. I am attaching the Technical Data sheet for the Nottingham Ale yeast , that I downloaded from their website.
As you can see, it has got a lot lot more information.
When the kit manufacturer's do multiple test batches prior to the release of a kit they always pitch the yeast dry and have equally great results as the same beer brewed using rehydrated yeast.
With all that said if rehydrating is a process that you’re very familiar with and have done successfully then feel free to rehydrate.
Hope this helps !