OK, answer us this: Can/will the Oregon accept the marine maps? If it can, I'd have to agree, why would someone NOT want the higher res screen?
Yes, it works fine with marine maps. You can install the same charts onto handheld Garmin GPS units as what you install onto the marine ones.
northoceanbeach: Last year you were cruising with a smartphone (I think it was an iPhone). If you stay with that plan then the iPhone can do duty as a handheld GPS while hiking and cycling. The primary advantage of the Oregon over the iPhone for hiking and cycling is that it has replaceable batteries, but that mostly becomes a benefit on multiday trips.
The helm-mounted GPS gives you another advantage that you didn't mention. You can get sonar integrated (and radar integrated if you want to get fancy). I like having a single easy display to watch for depth and charts. The fishfinder display is very helpful for understanding the bottom profile and makeup when anchoring.
It is easy to setup most of these smaller marine GPS units to be removable when you are away from the boat. My Raymarine e7d came with a gimbal mount that lets you remove the unit with two thumb screws and by unplugging 3 cables (power, sonar, network).
If you already have a handheld GPS then by all means use it on the boat. If you are buying new I think that a marine unit makes more sense for about the same price.
Finally: watch REI return areas for used handheld GPS's. I got the Oregon 450 that lived on my Catalina 25 for $50 in the Seattle REI basement. At the time the same unit cost $400 new at REI. There was nothing wrong with it, the note said "batteries died too quickly". I brought it home and looked at the one track on the unit, it had been sitting on someone's counter for 8 hours doing nothing.
This is where you can find free charts for the Seattle metro area that are compatible with Garmin handhelds:
They also sell charts for a lot less money than Garmin.