For wedding work my first choice is the 70-200 f/2.8 L IS Mark II. However if it should fail during that day, absolutely nothing wrong with having my 70-200 f/4 L IS nearby. Like others have said, knowing the the lenses "Range" will allow you to get a shot.
For years my f/4 L IS sat in the corner all by it self. Being an f/2.8 "Snob" the f/4 just sat and sat all quiet.
Then one day when the 50 plus old body wasn't as spry one day, I decided to carry my smaller, lighter equally impressive f/4 L IS. What a nice way to work without putting the load on me when f/2.8 is not crucial.
Good thing I never sold my f/4 L IS when many people told me in 2011 when I bought the f/2.8 L IS Mark II.
If you have the f/2.8 L IS Mark II and do not my the weight at all times, DO NOT Sell it, however if you are looking to lighten the load at times when f/2.8 is not desired, I can certainly recommend ADDING the equally impressive smaller, lighter f/4 L IS.
Canon 5D Mark III (x2), BG-E11 Grips, 7D (x2) BG-E7 Grips, Canon Lenses 16-35 f/4 L IS, 17-40 f/4 L, 24-70 f/4 L IS, 70-200 f/2.8 L IS II, 70-200 f/4 L IS, 70-200 f/4 L IS Version II, 100-400 f/4.5-5.6 L IS Version II, TS-E 24 f/3.5 L II, 100 f/2.8 L Macro IS, 10-22 f3.5-4.5, 17-55 f/2.8 L IS, 85 f/1.8, Canon 1.4 Extender III, 5 Canon 600 EX-RT, 2 Canon ST-E3 Transmitters, Canon Pixma PRO-10 Printer