Having just recently gone through this decision process building a hunting rifle for my son, I will add my opinion here. I wanted a light-weight bolt action rifle to build a 7mmBR for my son, who is 9 years old. I looked at the Remington Model 7, the CZ 527, and the Mini-Mauser. I chose the Mini-Mauser for some (what I feel are) very good reasons:
1- It really is a miniature action, not just a shortened action, so it is lighter and smaller than a Howa, Remington Model 7, or a 700 short action. Several ounces lighter than any of them. Ounces add up into pounds.
2- Since the action and magazine are thinner, the stock can be thinner as well. Remember, a lot of weight comes from your stock, so making it thinner and lighter will reduce weight nicely. Again, ounces add up to pounds.
3- No magazine box sticking out right at the balance point to clutter up carrying the rifle one-handed. The magazine in the Mini-Mauser has a drop floorplate for ease of unloading, but no big magazine sticking out right where you want to put your hand. And an internal box mag is lighter than a DBM.
So I opted for the Mini-Mauser action, opened the bolt face to take the 7mmBR, put a Green Mountain barrel on it with about #2 or #3 contour, slapped a Timney on it, reworked a ruined but nicely figured stock I had taken off someone's rifle because of their aborted attempts at stock-making, and put it all together. I wound up with a nice, compact deer rifle that weighs well under 6 lbs with a 3-9 scope on board. Not truly a lightweight, you say? True, but for a first-year hunter, I didn't want any recoil, and I achieved that. Plus it is accurate (about 1/2" @ 100 yds), extremely compact, and looks good to boot.
As far as usefulness of the cartridge, I see a lot of folks swooning over the 22 Hornet (which just baffles me), and I see people trying to hunt with everything from popguns to cannons. I say "more power to 'em". So, if the OP wants a 6.5 Grendel bolt rifle, go for it. It's a project rifle. It may never get changed, but it may get rebarreled and changed several times over its lifetime.