As you get higher level on the main game, it becomes easier to get normal gacha seeds... so use them ASAP to get the manyu, 3*, and more rarely 4* and ampies.

Also, some 2* units share skills with rarer units - for instance, 2* Margaret's Bludgeon is also on 3* Nymphaea, and 2* Yuki Yanagi's skill is on a 4* (the name escapes me right now). And 3* can often share with 4*. It seems like 5* and up are usually unique skills though in my experience?

At least until you have one of each 2* and 3* unit maxed at skill level 5 and 4 slots, and your 4* units like Hibiscus with skills shared with lower rarity units at skill level 5, it's better to use what you get.