Now there's a way to open up a nice can of worms...
Lots of opinions from lots of people on this type subject. First off, if you are simply looking for a new flat-shooting rifle for deer-sized game, then I'd agree that you likely do not need to move up to the .300 mags. There are a couple of flat shooters that will still stay reasonable in recoil, but allow you a bit more flat trajectory.
First one I'd mention would be the 6.5/.284, a newly SAMMId wildcat that is all the rage for long range target shooting, but is also an awesome long range hunting number too. Bullets are readily available from about 110gr to 160gr with 120-140gr being the most typical hunting weights. You can shoot a 120gr TSX, 125gr NP, 130gr Accubond or a 140 Nosler Partition (lots of others available too). All loads with good bullets in these weight classes will fly very flat and my experience with it's ballistic twin (6.5 RM) is that it's recoil is relatively mild. I'd recommend this cartridge to someone wanting to do their own reloading.
To my shoulder, and in like rifles, the 6.5/.284 recoils like a 165gr/.308 while providing 3100+ FPS with 120s and 2900+ FPS with 140s and all with excellent SDs. Next up for deer-sized game again would be the .270 WSM. Housed in a S/A (of course), it may end up being a tad more compact than some .270 Win rifles, while providing an extra 200+ FPS to typical .270 Win loads. Again, this is still a relatively mild recoiler and I find it no problem at all at the bench, similar to .308 or 30/06 shooting 180s, I'd say. This is an excellent, flat shooting number!
Now, if you're thinking game bigger than deer, then the .300 mags have a definite up-side. If you are not, they will still get the job done, and done very well, but will recoil a bit more than need be(?). I've owned two different .300SMs and collected deer with both. Neither buck was particularly far away, with one on either side of 100 yards, but boy they put a whoopin' on those deer, dropping both in their tracks (something that makes me smile). So, I could hardly argue with why someone would want to use a .300 mag as in my experience they work very very well. Personally, I'd prefer a .300 SM (or .270WSM) to a full length 7mm RM, as the rifle will be a bit more compact and recoil about the same in similar bullet weights (at least to my shoulder)