The so-called "bi-metallic" design of these blades refers to the fact that the blade itself is a different type of metal than the spot-welded plate the blade is attached to. It does not imply that the cutting part of the blade has two types of metal.
It would make sense for any high-speed power tool to make the blade of HSS steel to withstand the heat. But these don't. At this price, they could give you carbide-tipped teeth or at least induction-hardened teeth. Even Japanese HAND SAWS have induction hardened teeth... why not these?
Profit. Not just an honest fair profit, but exorbitant greedy profiteering profit. That does NOT mean these are bad; they are just overpriced.
Don't imagine they have you over a barrel though. The metal in these blades is soft enough that you CAN re-sharpen them! I do that with diamond needle files. Even with youthful vision, you need high-power magnification and good lighting. Nowadays, I have to use magnifying glasses AND a jeweler's magnifying lamp. It only takes a few minutes of careful filing and the edges come right back though!
One note: The "precision" Holz blade ( for wood ) pictured here on the right has a slightly different tooth-geometry than the other two blades. The others are very simple to sharpen, just like rip-cut hand saws, only much smaller. The "precision Holz" blade's teeth are similar to Japanese crosscut teeth, so they are more challenging to sharpen. If you've ever tried to sharpen Japanese saws, you will know what I mean. You can get small diamond files cheaply, but I have yet to find any file slender enough to file the "precision Holz" style teeth effectively.
I know some pro-remodelers who swear by their Fein MultiMasters, but they go through blades fast, especially when cutting abrasive materials. That's why I got interested in sharpening these things in the first place. I don't have much use for the oscillating tools myself, so I do not have much experience with them. But I do know this...
Now that the patent has expired on this type of tool, Fein no longer has its strangle-hold on the market. You might have seen the battery-powered model that Bosch makes or the Dremel powered model. What you might not yet know is that you can get a beefy powered model from Harbor Freight tools for forty bucks and it is remarkably similar to the power and durability of the higher-priced competitors. It isn't pretty and does not have the nifty quick-blade-change feature of Fein, but it cuts exactly the same.
Another money-saving tip... The cheap tool does not have nibs to fit the star-shaped holes in these Fein blades. If you attach them with screw-pressure alone, they have a tendency to slip.
No Problem! Just dab a little Loctite screw compound on the blade, screw it down firmly and wait one minute. The blade sticks well enough to hold in place, but will come off easily enough when it's time to replace it. Also, the replacement blades that Harbor Freight sells are substantially less expensive, yet are of similar design and quality... which is to say, not great. But hey, if you just need the tool once in a while, it works just fine... not Fein.. but FINE. (Sorry, couldn't resist ;-)
My buddy keeps TWO of these cheap oscillating saws on hand for renovating bathrooms and kitchens. Instead of switching blades, he switches saws. On one saw he keeps a metal-cutting blade for pipes and such. The other sports a larger semi-circular blades for tile and grout. At forty bucks a saw and seven bucks per blade, both saws and blades cost less than one Dremel or Rockwell version, and MUCH less than the good but expensive Fein. He keeps both saws plugged in and can switch between them faster than any quick-change mechanism can switch blades. He learned to sharpen some of these blades and doesn't even bother removing the blades to re-sharpen them. He just clamps the whole tool to his saw-horse, or holds the thing in one hand while he sharpens with the other. He has been using those tricks for over a year now... same blades, same cheap saws. No problems other than blade slippage, and the Loctite took care of that!
So... bottom line. If you are not a pro and don't need the quick change feature of the expensive Teutonic tool, get the cheap one and the cheap blades. Use Loctite Sumu-glue or Loctite Red thread-locking compound to hold blades securely.
If you are a pro and must have the superb Fein tool and expensive blades, get a set of cheap diamond needle files. They will work on most of the blades and you can save a ton of money on blade replacement.