I like this idea because:
a) It makes races unique.
It adds some interesting strategy elements into the cloth map.
That said, there are couple of issues regarding the whole proposal and/or the suggested implementation.
First, I don't think that we need more than 5 types of biome. True, the more biome specialists, the more unique races get, but there is a downside: with an increased number of specialities, each one of them will become more similar to others. For example: Mountain biome: +20% to def when fighting in mountains, +10% movement, ..; Forest biome: +15% to def when fighting in forest, +20% to movements, ... If there are 10 races to choose from, you will get aprox. 2 races for each biome, which is ok, at least in my book.
Second, the more biomes spec. you have, the more biomes you will have to implement. That leads to a world that looks like a chess-board (mountains-swamps-plains-xxx-yyy-zzz-mountains-...). On a chess-board every biome had to be represented fairly equally, in order to avoid balance issues. That, on the other hand, would affect the map's feel. In other words: the randomized world would feel very artificial, without large stretches of plains/forests.
Nitty-gritty part is as follows: 5 biome specialists:
- Polar deserts
I chose these five as carefully as I could, so that they would be balanced. Mountains & forests are a commonplace, however, they are only a part of the terrain - i.e., there might only be packs of them and they won't be always present when you need them. On the other hand deserts & their polar cousins are 'lands', that means they cover the whole 'land', thus if you find a few tiles covered by deserts/pdes. you can assume that there will be more valuable land for you populate. The downside is that deserts aren't as popular, as plains, jungles, etc. The unique category for themselves are the coasts. Coasts can be found everywhere, but it would be foolish of anyone to cap only coasts neglecting vast inner-lands.
I understand that due to game mechanics some specialities would be useful more often (forests) than others (polar deserts), but you can always balance the biomes; e.g., while polar deserts are rarer than forests, the latter doesn't give that great bonuses, while polar desert races would be the only ones to effectively populate polar caps without a danger of being harassed by other races (that would also want to claim the polar caps for themselves).