While not all that inconvenient, if you're new to the crypto space and maybe Splinterlands is your first experience in the space, the process of acquiring crypto with fiat money, and then swapping it around, and potentially paying high gas rates depending on the method and route you take to do so, just to get to having in-game currency can seem daunting.
Perhaps you just want to be able to invest a little here and there quickly and without having to jump through all the hoops. I know I did. I realized utilizing the ability to buy credits with fiat money was a convenient way to get money into the game, and then badda-bing badda-boom I could use those credits in the market to buy and sell cards to turn my credits into DEC. BAM. Magic.
I messed up.
You don't want to be like me and not do research first. I decided to just buy a couple cards that would soak up the credits I had purchased, knowing nothing about that cards history or market liquidity. I purchased 2 Lord Arianthus at whatever the lowest current price was, without realizing that there wasn't much liquidity in that card at that price at the moment, meaning I would have to take a substantial loss to hock it and get my DEC, or be stuck waiting for the market to hopefully come up to the price I paid for them. (As of now, they are still on the market and I am still regretting it, as the price has only come down from when I bought.)
Lesson learned.
While I don't see an immediate flaw in my plan to conveniently buy credits with fiat, and use the market to turn that into DEC that I can earn airdrop points with, be certain that you watch the market for the right card to use as a method. You want to use a card that has high liquidity (lots of people are buying and selling it in the market), that you're getting it at a good deal that isn't likely to drop in value, and that you don't squat on it if the price is likely to go back down (because there's a chance that, like me, you're just trying to inject money quickly into the game for FOMO and are susceptible to making rash decisions).