In silver league, the winner gets 39 fragments and 163.8k guild power. The 8th place loser gets 28 fragments and 117.2k guild power. The difference between 1st and 8th doesn't seem like it is worth the effort. But as has been pointed out, the biggest factor isn't where a guild places from season to season, but which league it is in. Being in a higher league means getting significantly better rewards, even for finishing last.
Any guild that finishes in last runs the risk of being demoted to a lower league and getting significantly lesser rewards. Some people find this demotivating.
Also, there could be a guild of 60 members but only one member actively participates in GBG, yet all 60 members will benefit - they will all get fragments and the guild will get power. With only one member participating, that guild will be in copper league so the rewards won't be that great. But if all 60 members were to participate - even at a minimal level - that guild would easily be in gold league or even platinum league. By not participating, those guild members can be grateful they are getting something for nothing, but they could all be getting so much more.
The copper league rewards are the minimum that any guild will get for participating. It's like those non-competitive children's leagues where all the kids get a trophy just for participating. Similar participation rewards can be found throughout the game - 1 FP per hour just for keeping the bar below 10 FPs, free coins from the town hall just for clicking on the building once a day, and so on.
Each guild has an MMR (Match-Making Rating). This MMR is originally computed using various factors, but after a few seasons the initial factors will be of no account and only placement within Battlegrounds will determine a guild's MMR. This MMR is used to divide the guilds into leagues.
Let's create a scenario with a world where there are 320 guilds that participate in Battlegrounds. Then the 32 guilds with the highest MMR will be placed into diamond league. The next highest 80 guilds will be in platinum league, then 96 guilds in gold league, 80 guilds in silver league, and 32 guilds in copper league.
With 32 guilds in diamond league, there will be 4 diamond battlegrounds. These battlegrounds can be thought of as diamond-1, diamond-2, diamond-3 and diamond-4. The eight guilds with the highest MMR are placed together in diamond-1. The guilds with MMR ranking 9-16 are placed together in diamond-2. The guilds ranked 17-24 are in diamond-3 while guilds ranked 25-32 are in diamond-4.
While there are 32 guilds in diamond league, those guilds will not all be of equal ability. In any world, there will probably be 2-4 guilds who are quite a bit better than those below them. Those guilds will likely be in diamond-1 and stay there. Below that highest echelon of guilds will be a smattering of guilds - maybe 10-20 - who are competitive. These guilds will mostly stay in diamond league, but will shift around between diamond-1 and below. Then there will be another range of guilds who will bounce back and forth between the highest platinum battlefields (platinum-1 to platinum-3) and the lower diamond battlefields (diamond-3 and diamond-4).
Nothing is fixed, however. If a guild loses a few members, it will probably drop to a different (lower) equilibrium point. A guild that adds a couple of power players is going to climb. How much a difference one or two players makes depends on where the guild was. A guild that was generally around platinum-8 might find itself around platinum-2 with the addition of a power player or two. Or find itself down around gold-8 with the loss of a player or two instead.
The gap in abilities between a guild in platinum-1 and a guild in platinum-5 isn't that big, but probably big enough that the platinum-1 guild would probably win a large percentage of the time in a matchup between them. Which is to say that the guild that takes 8th place in platinum-1 is still a stronger guild than most guilds that are in platinum-4 or below, and certainly stronger than any guild in gold, silver, or copper.
With the exception of the top 2-4 guilds, most guilds are not going to stay in the same battlefield for very long. A guild that places in the top 3 will likely move to a higher battlefield (platinum-5 to platinum-4 or even platinum-3). And a guild that places in the bottom three will likely move to a lower battlefield (platinum-5 to platinum-6 or platinum-7). Except for diamond-1, there should be plenty of movement of guilds from season to season. And even diamond-1 will see fresh guilds from season to season (although over several seasons, there will be repeats - "Hey, we fought against these yahoos two seasons ago").
What this boils down to is this: the guild that takes 8th in platinum-1 is equally as good as the guild that takes 1st in platinum-2. On that basis, there shouldn't be a whole lot of difference between the rewards they each get.
On the other hand, the guild that takes 8th in platinum-5 is significantly better than the guild that takes 1st in gold-5. It's just that a guild in platinum-5 is facing much harder competition than the guild in gold-5.