My question is, what are the pros and cons of putting descriptions like that on your official landing page? Is there a legal downside? Is there a marketing downside? Why don't people do this more?
IANAL, but in the US, I am not aware of a legal downside as long as you're careful about trademarks. You can use other people's trademarks in ad copy, for instance, as long as you don't do so in a way that could cause consumer confusion (people thinking that the company whose trademark you used is in some way connected with your product.)
Once you are a more established company, though, you really need to be able to describe your offering and differentiation independently of other companies.