Just did some quick math:
I currently have 245 max stamina and it takes 6100 EXP to level (level 66). If I spent all my stamina running ?-4 and used 1 gem to refresh, I'd be able to run the stage 12 times, with 10 energy leftover. Using the 3 gathers, I could get at least 1 more run. 13 runs of the stage would give me 6500 exp, ~10400 seeds, a good amount of gifts/gold, and level me up. Until the I hit a level where the exp needed/max stamina ratio gets too high or I hit really unlucky gathers, I can basically do this as long as I want. Also, as I level up I'll be getting in more runs of the stage per gem, since my max stamina will also go up.

5 gems spent this way would give me about 52,000+ seeds, giving me a good chance for a 4-star (or even 2-3 given the odds), as well as lots of other side benefits like gifts/gold and the extra prizes from 15 gathers. This is worse at lower levels when each gem refills less stamina, but at lower levels you can often level without even spending a gem since the stamina you gain from gather has a larger impact. This is even better at higher levels (assuming you can still level by spending 1 gem) because of the higher max stamina pool.

Not saying that you should blow your gems this way, and this is using an event stage that won't always be here as an example, but it's something to think about. Currently, if you're rolling premium the only winning rolls are 5/6 stars, everything else is pretty much a losing prize.