I haven't spent a cent and I won't. For me (and only me, I know others have different goals), spending just makes an already-too-easy game easier. It's like buying experience maps and such in the 3DS Fire Emblem games to take out the challenge.
Heck, I can even stay in tier 20 sometimes if I have the right bonus hero that week. If I'd focused on getting merges instead of getting 7+ teams of units ready, I could easily stay in tier 20 all the time. The only thing stopping me is the silly points system that favors merges so heavily.
Speaking of which, the fact that a ftp can stay in tier 20 means that either there aren't very many whales, or a lot of whales don't care about the arena.