Benefits or disadvantages aside, you can't import JPEG as DNG in Lightroom (to my knowledge). If you import a JPEG file in "Copy as DNG" mode, LR will prompt an error dialog saying, in substance: "Non-raw files were not converted to DNG". What you can do, however, is select your JPEG file in LR, and export it as DNG, then re-import that DNG file.

DNG is really designed for RAW sensor data. For instance, your typical DNG file doesn't contain a color space. Its data structure, bit depth and format are very different from the RGB pixels found in a JPEG file. Think of a JPEG file as a "baked" version of a RAW file.

There is a "Linear DNG" variation that can store a demosaiced RGB image data arranged in a rectilinear format. It is supported by LR and is used for cameras with Foveon sensors for example. However, you do not increase the quality of your JPEG file by converting it to Linear DNG. Everything you can do with a Linear DNG you can already do with a JPEG file in the Lightroom UI.

As far as metadata and develop adjustments are concerned, they are stored in Lightroom's catalog file, unless you tell LR to export them automatically, or do so manually by selecting "Metadata -> Save Metadata to file". If your photo is a JPEG file, LR will create a XMP sidecar. If it's a DNG file, LR will be store these settings directly within the file itself. At this point, yes, you can say DNG is a bit more convenient, especially if you want to share these settings with somebody else. Personally, I do not let LR export metadata automatically for performance reasons, and I rarely save the metadata back. What I make sure to do, however, is to carefully backup my catalog so that these memories we are preserving are not lost. Look for "lightroom backup" on this site (or check this one specifically).