It's simple... Buying tokens near enough guarantees you a League win, and significantly increases your chance of winning the CL as these are both seeded by Manager level.
Buying tokens significantly decreases your chance of winning the cup, as you will be seeded by team Quality, therefore if you have a scout team you'll be seeded with teams that are a higher level, and as such, will have access to better players.
From what I've seen, the teams that have the best chance of winning the cup, are the ones filled with low Q 5 star players, ie. quality ending in 1 and 6.
I've bought tokens since day 1, and I won the cup in my first season as I was already seeded against level 1 managers before I bought half a squad of scouts.
I didn't win it again until season 11, where due to financial difficulties I couldn't buy tokens, my average quality dropped to a high 4 star, and lone and behold, I won the cup.
I'm happy with having plenty of pretty red stars in my team, and resigned to the fact my chances of winning the cup vs. teams 3/4 levels higher is <5%.
Doesn't help that the majority of players on my server are Indonesian/Vietnamese etc. where tokens have proven to be cheaper, and the majority of 'rich' token buyers lie, part of the reason I hardly ever use the regular transfer list anymore.