The quick answer to this question in a pinch is, yes, a rotary tool, such as the Black & Decker RTX1 or the Dremel range, do come with a cutting disc and grinding stones and can be used for cutting or grinding metal, rigid plastic, wood, composites, etc.

The real question is, how economically viable would it be for you to use a rotary tool for the task at hand? In this example, we see a Dremel being used with a cut-off wheel being used to remove a rusted clamp on an automotive drive shaft boot.

Scenario A:
Assuming you already have an existing rotary tool and simply need to cut a handful of rusted and hard-to-reach clamps, then all you would need to purchase is a metal-cutting attachment, such as the Dremel SC406 starter set which includes the EZ SpeedClic mandrel and two fiberglass reinforced metal cutting wheels for around RM35. Most rotary tool uses a collet chuck and are compatible across brands.

Once you have the EZ SpeedClic mandrel, you would only need to purchase the Dremel SC456 cut-off wheels if you intend to use the rotary tool for future metal cutting projects. The SC456 costs around RM22 for a set of 5s 38mm discs.

Plus, with a flexible extension shaft attachments such as the Black & Decker Wizard Flexible Shaft RT5100, you could even use it in extremely cramped areas for precision cuts - without damaging nearby components.

Short-term gains.
Obviously, going this route is cheaper than forking out four times more for a new angle grinder with a cutting disc in the short term, however, if you do plan on cutting metal in the long-term or if you have a big project on hand, then it's far cheaper to purchase an angle grinder outright rather than have to deal with small cutting discs.

The cost factor.
Despite the size, disc-for-disc, the Dremel cut-off wheels costs about the same as a brand-name 4-inch cutting disc from Pferd, Bosch, Hitachi or Makita. However, due to the small 38mm diameter size of the cut-off wheels coupled with the high-speed of the rotary tool (5,000 to 30,000rpm), you won't get a lot of cuts out of this one attachment so the cost goes up!

In terms of thickness, the Dremel cut-off wheels is also slightly thinner at 0.75mm when compared to the majority of angle grinder cutting discs at 1.0mm, with the only exception being Makita's 0.8mm cutting discs. Obviously, the thinner the disc, the quicker and more accurate the cut - the downside, is thinner discs wears faster.

Scenario B:
If however you have the luxury of space, and need to cut off rusted parts on a regularly basis, then it makes more 'sen-se' to actually purchase a dedicated angle grinder with a cutting disc for the job. the 4-inch cutting or grinding discs are fairly common and are usually a staple of automotive garages for this particular reason.

Plan C..?
In an ideal world, you would have both tools at your disposal as both tools are specific for the type of task they were designed to do. You should also consider other options, in this case, the rubber boot is probably cheaper than the Dremel cut-off wheel and they are are wear items that needs to be replaced anyway, so a simple hacksaw in your tool kit or a wire cutter would work just the same. Unless you're just repacking the drive shaft with grease, chances are, the rubber boots would have to be replaced anyway.