Some macro lenses have a really nice focal length that would make them a nice prime-telephoto lens, but are there any downsides of using a macro lens when shooting distant subjects (besides the lack of zooming)?
Most prime macro lenses are suitable for distant subjects. However, there are some exceptions:
the king of macro photography, Canon MP-E 65, will not focus far enough to fit more than an eye or nose on a portrait;
some macro lenses, like Pentax DA 35 Limited Macro, have a short focal length -suitable for distant subjects only as environmental shots showing context rather than details of the subject; shorter than about 50mm on APS-C or 75mm on full frame are generally not considered suitable as portrait lens;
some zoom lenses are also sold as "macro" lenses; generally they have a variable aperture similar to consumer zooms. You can take portraits with them, but you have to use other tricks to get a good background separation (e.g. background far away, plain background, lighting subject to underexpose background).
Macro lenses are made to be comfortable for precise manual focusing (because that's how macro is mostly done), so their large focusing range is spread over almost a full turn of focusing ring. This implies that auto-focus can be a bit slow, especially if there's no focus range limit switch and the lens goes hunting through the whole range. Prefocusing to an approximate distance might help you here in many cases.
Another disadvantage in using macro lenses compared to a regular primes lens is their moderate maximum aperture for a prime of similar focal length (especially ones preferred for low-light, fast action or portraiture), usually in range of f/2.8 to f/4.5 - for macro, more would be overkill. Tamron 60mm f/2.0 is a surprising exception here; unfortunately 60mm has to be so close to subject it will scare away living critters, also lighting becomes challenging; so it has somewhat limited use in macro world.
The smaller aperture means less flexibility in getting thin depth of field. But small maximum aperture means the aperture for maximum sharpness is even slower (typically by a stop or two), meaning you have to take harder compromises between sharpness and background separation by DOF.
That said, an f/2.8 macro lens is still on par with professional zooms aperture-wise.
No disadvantage, macro lenses generally make good portrait lenses, especially if they're fast. A macro lens only differs from a standard lens in that they can focus at a closer distance.
Yes, yes, yes, and yes. Macro just means that the lens has a very short minimum focusing distance and can reproduce an image with a 1:1 ratio at a certain focusing distance range. Outside of that, a macro lens can perform just fine as a regular lens that doesn't have macro capabilities. I love taking portrait pictures using my 85 mm f/3.5 macro Nikon lens.
Nikkor's 105mm f2.8 is commonly used as a portrait lens.
As I searched more info about macro lenses - especially Tamron AF 90 mm f/2.8 SP Di Macro, I read a lot of reviews and the guys at Lenstip say "Lenses of the focal lengths ranged 85-105 mm are ideal portrait instruments often used in macro photography." - similar opinions were also in other reviews.
It is better to use a macro lens as a tele-prime than the other way around, if you don't need a larger aperture than the macro lens provides or super fast AF. The difference is in the optical and mechanical construction.
Optically, the macro lens should be made to give flat field focus plane, while the prime is curved (by equal distance from point to lens). The macro lens is also constructed with floating elements to correct for spherical aberration, distortion, and preserve sharpness as you focus closer and closer, whereas normal lenses might be made very simple moving the entire set glass elements back and forth (premium glass does have floating elements as well), and if you use them with extension tubes you are focusing by moving as all further away, and thus increasing the projection of the image on your sensor, leaving no control to fix any aberrations, distortion, or blur, i.e. magnifying these optical issues.
Mechanically, the drawback of the macro lens used as a prime is its strength when used for macro: It is more for precise manual focus, which means you change focus slightly with a big movement, instead of having the entire focus range on a short distance. Good for manual focus, bad for AF chasing distant and close subjects dynamically.
Is there a drawback in using the flat field sharp macro lens instead of a prime? Well, if your prime option is a F1.2-1.8 and your macro option is F2.8 and you won't get good background separation, then it is. F2.8 at 20cm distance is really narrow but not so much at 2m. Also for portraits if may not be flattering to see the skin too sharp, so a soft prime with a glow wide open might look better.