The poor rating is based on what I think of the product in it's original form and not my intended use, for which I think it's great.
I've had one of these since many years and ordered one more since I have one more guitar now.
The general idea behind the product is good, even though it's not revolutionary. The main issue with the product (and others like it) is that the sponge can hold very little water.
Depending on circumstances you will have to wet the sponge at least once or twice a week, IME.
If you intend to use the sponge bundled I'd suggest you at least get the "pro" version instead, that can hold more water, or even look at another product.
However, I didn't intend to use the sponge and use it with orbeez/water pearls instead (it's quite an easy mod), so I bought the cheaper alternative.
So, the plastic "case" is, IMO, useful, since it is easy to modify for use with orbeez and it can sit in the sound hole while the guitar is in the case, so you don't have to mount it to the case itself or worry it might scratch your guitar.