Like Umbreon Dan said, there are already existing examples of billymills' concept in today's metagame. If you really think about it, you have all sorts of Pokemon that can fit one role extremely well, and fit another role extremely well, but can't fit both roles extremely well. Aside from Heatran as an example, here are some more examples:
Cyclohm:
This CAP can be used as a fantastic offensive Pokemon, and it can be used as a fantastic defensive Pokemon. However, if you attempt to make it go mixed, Cyclohm will a) be significantly weaker offensively and b) take hits much more harder than usual.
Jirachi:
Jirachi fits this concept as well, both offensively and defensively. Scarf Jirachi is a pain in the ass to get rid of, while SubCM Jirachi is a pain in the ass to get rid of. However, Jirachi can't necessarily run both of these effectively in the same moveset, since it will be pretty lackluster. As for the defensive side, it can take physical hits like a champ, or special hits like a champ, but not both at the same time.
Empoleon:
Last example. Empoleon has a pretty great offensive stat spread, 86 base Attack and 111 base Special Attack, not to mention that it has Swords Dance. Empoleon's popular SubAgility set is insanely threatening after pulled off, and can easily sweep unprepared teams. SDPoleon can wreak havoc with boosted Aqua Jet, Waterfall, Earthquake, etc. However, it will never be effective at running a mixed version, because it just won't be strong enough.
Like billy's concept states right here:
Can play two incredibly different roles extremely well, but no middle ground. If this pokemon attempts to compromise its strengths, it will suck. Ideally, it will absolutely destroy one type of pokemon using one of its strengths, but will be completely countered if it chose the other specialty. Important: It has no true 100% counter.
If the above examples were to compromise their strengths, they will suck.
As of now, there are no Pokemon that follow moi's concept. There are definitely ones that can laugh at certain non-damaging moves; like Gastrodon & Trick, Breloom & Leech Seed, Clefable & Status, but there aren't any Pokemon that can laugh at a
variety of these at the same time.