Combining 2 soul cores is not the only way to obtain 'very rare' creature soulcores, you can also get them by using soul prisms or exalted cores on any soul core. Monsters that are of classification "rare" cannot be fiendish, therefore they don't drop rare/very rare cores.
Only creatures whose occurence is classified as 'common' in cyclopedia can be fiendish creatures. When killing them, the player have a small chance to drop the soul core of the fiendish they killed and an extra extra extra slim chance of dropping these special cores of creatures classified as uncommon/rare/very rare that you're referring to.
As for if it's 'worth' it, generally speaking, combining soul cores is the last of the options one should choose because you lose 2 guaranteed soul core's profit into 1; and you have a chance to turn 2 "average" soul core into a really bad one (harmless ones for example). So only do it if you don't have any other option at hand or if you looted the cores yourself and don't mind missing out on profits.
From my experience I've only done that in a situation where I didn't have soul prisms available and I had 2 soul cores of creatures of difficulty "harmless" while looking for a hard creature soulcore. If I used soul prisms, I would need to upgrade 4x in order to get there. But knowing that soul prisms supply is not infinite and rather rare, I would combine 2 of harmless to try my luck into getting a hard or challenging creature without wasting precious soul prisms. But in the end, it's still a gamble.
PS: The other answer stated that combining soul cores is the only way to obtain the ominous one, but there are many reports of people who got it by upgrading/downgrading a soul core instead of combining. One example is from this post: https://www.tibiaqa.com/36101/how-to-get-ominous-soul-core-does-it-have-any-function?show=36101#q36101
Sources used to confirm information: TibiaWiki Fandom and my unhealthy experience of rolling thousands of soulcores in an attempt to have the whole animus mastery completed.
