In the past I've bought diamonds when they had a "gift-your-guild" promo not because I needed diamonds, but because it's the only way I could reward my guild for good performance. However, the drawback is it rewards all guild members equally. As most guilds do, we have some overperformers and some underperformers.
If there was a "gift shop" where a random assortment of things was available each day, for a range of prices (say like 1.99 to 9.99 depending on rarity or usefulness) that you could purchase and then designate the guild member's name whose mailbox it should be delivered to, I would totally make use of that. I might not buy a 49.99 diamond deal but I could easily buy two 4.99 items twice a week to reward the top performers in guild war and crypt each week. Psychologically it's a lot easier for the average player to justify multiple small purchases than the big 49.99 or 99.99 deals, regardless of the bonuses they offer. I mean, if Gree only wants to target revenue from whales, that's their choice, but I feel that they're leaving a lot of money on the table this way. Take Pokemon Go as an example. They have multiple spend points; you can spend as little as .99 at a time if you want. 11 months in, while they've slowed down, they're still not hurting for cash in any respect. See for yourself: https://thinkgaming.com/app-sales-data/130634/pokemon-go/
Gifting like this could encourage guildies to play more/perform better, and in turn might even encourage them to spend a little bit themselves to better their own performance. So it's win/win for Gree and the guild.