I imagine the seller used another messaging app previously.
I use GoSMS pro, so longer messages are no longer converted to MMS messages... yay! And it's free. With the SimplePaper theme, and some font sizes decreased, it looks really nice, and could pass as stock, unlike Hancent (in my opinion).
Since I get on so well with it, I used Titanium Backup Pro to freeze the stock app... This prevents it running, and hides it from the system (saving ram, cpu etc. I guess) - yay, tidied up! - I guess the original owner also did something simillar... He may have removed Bloat Freezer, or Titanium Backup Pro, but it's effects are probably still left behind.
You could get TBP or BF and see what is frozen maybe?
TBP is better imo, as it backs up your phone, everything on it, apps, app data, and settings on a scheduled basis... I have mine back up every other day. if anything ever goes wrong, I have a max of two (set in settings) backups of the whole phone to restore... so if I drop it and it breaks, or get it wet and it needs replacing, or needs to be swapped for warranty, I can press a couple of buttons and put the whole phone back to how it was before... Was also very useful when I upgraded my firmware, and somehow lost everything!
Highly recommend keeping it rooted, as well as geting Go SMS Pro & SimplePaper theme (both freeeeee!) as well as getting TBP
By removing (sorry, FREEZING... personally I would not remove them, but TBP does give you the option to) all the hubs and samsung crap you can boost the battery life quite a bit I found. Especially after freezing Wi-fi sharing... strangely, even after being frozen, wi-fi sharing still works when switched on, but it stops eating your battery.
By keeping the phone rooted and installing bloat freezer, or titanium backup pro, you can freeze these apps and stop them running, saving CPU, RAM, and battery drain. Guess the original owner did the same with the stock messaging app... then removed the app he did it with before selling it?
Matt