Practicing with your 5-man premade? In a scrim? Playing a custom game among friends at a LAN party (do people still have LAN parties)? Let 'em catch up. It'll make the game more fun for everyone involved. Hell, in custom ya might as well restart the match.
In a normals/draft match? Play to win, unless EVERYONE on your team wants to let the enemy 'catch up'. For fun. For science. For sportsmanship. Whatever. But your team should agree.
Ranked. Play to win. Doubly so. You're there to compete.
What about in LCS? If someone's Ethernet cable/port takes a giant crap while on stage, officials will pause the match. Unfortunately, we can't do that in normals/ranked with an AFK. So the fairest thing to do for all involved is continue playing to win.
One way to look at it:
I play racquetball.
Lets say I've jacked up my right hand/arm. But I still wanna play with my buddies. We might agree that everyone plays with their non-dominant arm. Or that my buddy will take it easy on me because my arm is screwed up (or vice versa if his hand is bandaged). These are friendly, gentlemanly games. We want things to be fair.
But let's say we're competing for a club title or in an amateur's tournament. I'm not going to expect anyone to take it easy on me, and I'm not going to take it easy on them. If my hand's jacked up, I'm gonna bandage it and play through the pain anyway.
Alternately, you're quarterback at football practice. You're shoulder is screwed up. Your teammates take it easy on you because you're not in tip-top shape. You work on other things like plays, strats, etc because you're healing up. But when you injure your shoulder during a playoff game? You demand a giant lidocain injection from the team doc, and you play your heart out. The other team may be a great positive team, and show good sportsmanship, but I guarantee they will NOT take it easy on you. High-school to NFL, it's the same.
Same applies to league.
The way I see it: picking off a weak enemy isn't any more or less sportsmanlike than double-teaming the fed enemy or even 3, 4 or 5v1-ing them.
It's nothing personal, it's just good strategy.
While it may seem like good sportsmanship to let them catch up, it's bad sportsmanship toward your own team. And the thing is, you never really know why they AFKd. Power outage? Temporary Rage quit? Had to take an emergency bio break? Bad Internet? Not your fault, not your problem.
teleported to me at level 1 when i was 6 so i made a cease fire agreement with them. i would not kill them if they did not attack me. i let them catch up to 8 and i was at 12 then we went after each other.
and his 
jungle were level four while his opponent, an 
, was genuinely level 1 still. Granted this wasn't a case of the Ekko being AFK and it was just playing well on Faker and his jungler's part, but still -- they played competitively because that's why you play this game.
