What is there difficult to understand? You want to use the phone, you take it out of the case. You are done using it, you put it back. Doing each thing takes like 2 seconds, if that is 2 seconds too many for you, then i guess yes, it is inconvenient.
The upsides are obvious. You get to use and handle the phone in all its beauty. The big part of added value of Apple´s products is the premium design and feel of their stuff, you literally pay hundreds of dollars/euros/whatever for that, only to hide that right away inside some fugly bumper case. What sense does that make? Might as well buy then something like Pixel with plastic body, it is not like it does not get the same job done for the most part.
And yeah, the phone is “vulnerable” aside of transportation. But why should that be an issue? Just be careful with it and you will be fine. Granted, there are people, who are like naturally careless with their stuff, treating 1000 bucks devices same as 1 dollar pen, throwing the phone into bag or pocket just like that with some other stuff, dropping it on the first day, etc… i had a colleague like that. Safe to say, this sleeve is not for people like him. Me, on other hand, never dropped my previous 5S, i 4 and half of year of usage. Not just once.