When you put the top title on a guy who has only wrestled 2 or 3 times in the last six months it is obvious that there is little concern for that in the story lines.
You're right, but even waaaaay before Lesnar got the title, it became irrelevant. CM Punk's reign was ridiculous because the WWE booked Cena stronger than Punk and Punk's title matches in some occasions weren't even the "Main Events" of some PPVs.
We also live in an age where most World Champs have held the title 10+ times, because it constantly changes hands every few months! The WWE has also put the title on people that have constantly "not been ready" like the Miz, Del Rio, Sheamus, etc. Characters that don't have "deep history" like other World Champ have.
Lesnar, while not around, is still a "credible" World Champ. He was a MONSTER during his first WWE run. His character was well-handled and written (thanks to Heyman writing Smackdown at the time) and he not only has his WWE history, but he was a UFC Heavyweight Champ and NCAA Wrestling champ too. He also broke Undertaker's streak. Those are some THICK credentials and he's part of the OVW "Class of 2002" that brought out fellow champs like Randy Orton, Batista and John Cena.
Even when he's not around, his presence is still "felt" thanks to Heyman.
Having Reigns beat Lesnar would be ridiculous (even though that's probably going to happen)...Reigns is too green and he doesn't have the deep-rooted "history" that most contenders do. If WWE was smart, they'd throw John Cena (as US Champ) and Seth Rollins in the WM Main Event and have some sort of match where the World and US Titles are on the line. Have Rollins win the US (and feud with Reigns later) and have Lesnar (if he does indeed resign with WWE) win the World or else have Cena win it. And build for a climactic Brock vs. Cena Summerslam battle.
But WWE doesn't work that way anymore...