I agree to a point but we're talking about your chassis here. If that one point fails, you cannot drive the car if the cross-member comes apart in the back. Plus there is a MM right there, it's a highly stressed area.
Also having something chase out 10x1.25 threads comes in handy, trust me there. The money I dropped on Bolt Extractors, Carbide Drill Bits, and Tap/Die hand tools wasn't cheap, but I have since run into so many jobs where I used them again. Damn bolts that hold up exhaust hangers, for example, three bolts broke off super-easy on my 2nd car. NP, drilled a pilot hole, used a tapered extractor bit, chased the threads, put in a new bolt (x3).
Hopefully you won't need to use the inserts anywhere else, so ya if you can find someone with just the spare inserts you could do the rest if you had a right thread pitch tool to chase the threads. But the O.D. of the insert is going to be larger, the inside is 10x1.25 I think for these bolts, so you need to cut new threads which means another tool - kit may be a lot cheaper in the long run unless you can hook up with someone else.
Another alternative still is to just cut new and larger diameter threads, put in a quality bolt, that's where someone with a good set of tap and dies could save you all the cost except for a bolt. I bought OEM bolts for my broken off bolt (I was able to save the threads) but honestly see no reason why any comparable flange bolt with the right threads of decent quality wouldn't work, and that would cost all of $5 or less.
I hear you, throwing money at a kit isn't fun, but there are several other places where you have the same size bolt - mostly things that bolt onto the chassis like the exhaust hanger stuff, other candidates for breakage.