I don't use monsters often in my party, except maybe early in the game. I would have to say, though, that monsters are definitely worth getting. They serve a noble purpose, and it's called breeding. If you recruit even the simplest monsters and let them breed, they will beget stronger offspring eventually. Bull Demons will eventually breed Sacreds, and Yellow Chocobos will give you Red ones eventually. Morbols and Flotiballs are other wonderful groups to breed.

The point to breeding is that the stronger monsters (apart from being somewhat useful additions to your early party) will yeild some unique and sometimes powerful weapons and other items when you poach them with Secret Hunt. Sacreds can give you Holy Lances and Ivory Rods;Red Chocobos could give you Barettes;Plagues give you Yagyu Darkness stars and Zorlin Shape knives. Great Morbols are the best source of Elixers in the game, although on rare occasions they could give you a Mademgen dictionary. The more powerful monster types: Dragons, Hydras, Behemoths, and Uribos will become available as the game progresses and their poached rewards are even better.

I think people ought to get a couple of types of monsters early in the game and breed and poach the most powerful species. You will not be able to buy the poached items at a Fur Shop until Ch. 3, but the skins you poached in Ch. 1 and 2 will still count. Or just wait until Ch. 3, but make sure you still have more than a half-dozen character slots open in your party. Really successful breeding takes a lot of time and patience, but I.M.H.O. it is worth it.