Well by the letter of the RFC browsers and middle boxes (all the invisible caching proxies out there at your ISP, etc) are only supposed to cache if the cache/expires headers are set for that, but 200 is a bad choice for most people running a site for the other reasons listed above like crawlers/indexers. 5xx is correct for a problem on the server, usually 500, 503 or 504 (http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html).
At the same time there are many erroneous implementations (intentional and unintentional) that, as you imply, just cache any 200 without checking cache headers. This is also a good practical reason to avoid doing this.
At the same time there are many erroneous implementations (intentional and unintentional) that, as you imply, just cache any 200 without checking cache headers. This is also a good practical reason to avoid doing this.