Funny since they are antagonists of each other - you might work them in a way but you will not strengthen... as in the way the OP seems to intend by his question... it would be better to do the flexors and the extensors with their own exercise. He was asking on the usefullness however - and as I said - if his forearms are stronger than the resistance of the grip device - it will be useless. Which still stands.
... and you can ask whatever doctor you want z1ggy - but claiming someone is a doctor doesn't mean they are a good one... not saying Socially Challenged is not - just your logic is foolish... and while I know that I do not have as much training as probably a PT... it doesn't mean I have none - which you are assuming.
They are antagonists, yes. However, against what is typically taught for clarity, antagonists can and do contract at the same time. Frequently, it is taught (especially with the myotatic reflex) that when an agonist is contracting, the antagonist is inhibited and thus not contracting. However, that is a basic reflex. When the brain is actively completing a movement, agonists and antagonists regularly contract with little inhibition of one another.
With the gripping force required with a heavy deadlift, both the wrist/finger flexors and extensors are contracting heavily (sometimes maximally). It is not done in the typical concentric/eccentric fashion, but instead an isometric fashion. Isometric exercises prove to be very, very effective for strengthening (although you do not develop as much proprioceptive awareness due to the lack of strengthening through the ROM). With a gripping device, you're right. If the device is only 50% of your 1 rep max for gripping, then you're not going to get much stronger. If you apply the general concepts of strength training, you'd ideally want something between 80-90% of your max and do low reps. That's why they sell progressive grip trainers (easy, medium, hard) or even adjustable grip trainers (adjustable from 50lbs resistance up to 250lbs sometimes).
The act of gripping objects themselves will strengthen one's grip, especially if the other exercises are progressive. For example, let's talk about deadlift. If you're deadlifting 200lbs to start, your grip may not be the limiting factor. However, if you progress 5-10lbs per week, in several months you'll be at 300lbs. The isometric force generation required to hold onto that is fairly high. As I mentioned previously, grip training has to be progressive just like everything else. If the deadlift weight (resistance) continues to go up, then the grip will continue to be challenged at a high percentage of your grip 1RM and get stronger.
Also, it doesn't just work in theory/research. Anecdotally, I only did heavy compound movements as I suggested earlier. My deadlift went from 275 to 390lbs. I was able to do a weighted pull-up with 135lbs (at 155lbs body weight). In that time, I measured my grip strength with a grip dynamometer. It increased from roughly 100lbs (psi if I recall correctly) to 130lbs. I think average is usually 80-110 for males. Isometrics work in nearly all cases, but especially with grip.