Yes, because every card you win from FGs is upgraded when you win it, so you either save the upgrade cost or can sell it for 1100+ electrum if you don't want it. Also, unlike most other AI/arena levels, FGs are predictable because they have predesigned decks, rather than random (like halfbloods) or ever-changing (like arena).
I'm not sure whether FGs or platinum arena is more profitable. Arena definitely gets you more score, because you're getting hundreds of electrum per win (over 400 electrum for platinum EM) while FGs get you upped cards, but an EM is only worth 120 electrum.
Also remember to use the oracle FG predictions. If you know a certain FG is coming, you can usually build a deck specifically for that FG to wtfpwn it. For example, wings wtfpwns serket because he has no flying creatures, sanctuary wtfpwns decay's pests, gravity shield shuts down a few decks almost completely, and so on. For some of them, you can win fairly effectively even with no upgrades, or just a few.
About RoL/Hope: A lot of people swear by this, but it's actually not a very good deck. If you have few upgrades, it's probably alright to start, but I tried building a fully upped RoL/Hope deck because everyone raved about it, but it won way less than my rainbow control deck and was far less enjoyable even when it did win. If you don't care if your games are boring, Instosis has a fantastic win rate (like 70%) but it's no fun at all to play, because all you're doing is spamming sundials and hourglasses. If you've got very little electrum, I can see starting with it, but if you've got lots of upgrades already, don't waste your time with RoL/Hope, because you can build decks that win twice as often or more and do it faster if you have fully upped decks.
