I personally don't like circles as much as others, things like tear shape (not in demo) have circular properties and are interesting in different ways, triangles are cool too in their own way. Most shapes have a couple unique properties or techniques. The combination modes like rect+circles and plus+pentagons are a bit awkward admittedly. Circles are less "puzzley" feeling than the others since they fall in annoying ways constantly compared to more stable shapes, and rotating doesn't do anything with them so they are missing one of the strategy elements. Capsules are another semi round shape to try, they're somewhere between rectangles and circles for how they play.  

I didn't "fix" what you are saying, it's just what I decided to go with for this game, new shapes (spawned from merging or recently dropped) don't instant end the game but old shapes do if their center point is above the front box line. I've seen other games do different but the balance would be thrown off a bit too much if you could just spam shapes for too long at the top in my opinion. All the shapes were balanced rather carefully so you can "clear" the mode when you merge two suika/watermelon.

That being said, the upcoming update will have a lot more content for all shape modes including circles, because I am adding new stages very soon (crunching on it currently and almost done) and online co-op at the same time. The stages have a couple mechanics to encourage strategizing together. Circles of course is the most popular since everyone sees it as the standard mode, nearing scores from 1000 players. Random with all the shapes has consistently been popular too, since it offers a lot of gameplay variety