Here.s the problem that I see...
While your method would work for people that live close to eachother, many NES geeks live all around the world, and if you made a game based on the exchange of data, people it would be ideal to be able to share that data with friends all over the world.
How would you get the data to them? You'd need a way to interface your storage device with a PC, upload the data to a hard drive or other storage card, then e-mail it. This means creating a USB link between your dongle and a PC, which may be as complex as using an SD card on the game cartridge.
Id you want to go the route you are planning, you should investigate the Famicom 'Battle Box', which was a device for storing FC game save data after power-off for games that didn't include battery-backed RAM. I'll have to look at its schematics to see how it did it, but you can find them on-line to view yourself as well. (If you can't, PM me and I'll send them to you.)
It only worked with titles specifically designed for it, and there way no way to extract the game data from the deice externally, but you could plug it into another Famicom, which is essentially what you're talking about. The ability to save game data this way and share it is nothing new, but being able to send that data to another person or interface the NES to a PC in a manner conducive to doing so is (thus far) relatively new.
Only the PowerPak can do it, thus far. (in fact, it would be possible to create a custom mapper with a serial number and sell the game as a ROM that required a custom mapper, generate a mapper file for the PowerPak with an encrypted serial key in its source--randomly generated for each single one sold--- then track the serial numbers to prevent piracy and do this entirely on the PowerPak. Just a wild idea, probably due to lack of sleep...)
As to SD cards being frail, people trust them to gold photographs taken on vacation, and they are common and easy to replace (inexpensively). It's a simple manner to back-up whatever you have on the card and it you lose it, or it is damaged, you can replace it. If you design a device with internal memory, and it develops a fault, then not only will you still lose the game data, but you'll have to take it apart, desolder a SM memory chip and put a new on in its place.
Additionally, SD cards are pretty tough. I've only had one die. It wqs a Sandisk USB2+ folding card, and I accidentally snapped it at the folding seam when it was sticking out of a device, severing the ground connection. I recall that I lost another one that became saturated in Coca-Cola and developed corrosion, but i can always clean its contacts if I ever need to do so.
They're really tough little buggers, and as long as it's fully inserted into something, I don't see how you'd break one. (The folding card is an exception, because, well, it was hinged and I bent it the wrong way.) You may lose the card, but as long as you can plug it into a C (or Mac, or Solaris station, etc.) at the end of the day, that isn't a problem.
If all you want to do is develop a game that uses an external device to hold game SRAM data, but you don't want to share it across a network,(which will severely limit its usefulness, and your sales these days), you should essentially clone the 'Battle Box.' That's all it was, and that is all you'd need.
If you want to ensure that people can share the data on the Internet though, it will require connectability of some sort, and if you want to make it easy, it needs to be SD or USB. Anything else will limit who can use it.
Many systems today have only USB and micro-card ports, so serial is out, and if the USB controller doesn't recognize it and its data natively (using built-in or existing software drivers), then you'll need specialized software to view and extract it, which you'd have to make multi-platform (PC, Mac PPC/x86 dual-binary//Linux) in order to ensure market durability.
That's why, in my eyes, SD makes the most sense, but 'tisn't my project, so don't feel any pressure from me, unless my word that if I couldn't plug it into a Mac and share it with my mates means I wouldn't buy it is meaningful.
(Just about all of my mates that play NES/FC games live a long distance--500-600Km--away from me...)
I only have three local friends that do, and they don't own the console themselves, but visit on occasion to relive past memories. I'm pretty sure this isn't too uncommon, considering the age of the console, after all.
-Xious
UPDATE: I looked at the schematic for the Battle Box, and it indeed is exactly what you need to view, as it uses the $4017 register signals available on Port II of the NES. If you improve it to add USB attachability to a PC, you can pretty much use it as-is.