Bottle Conditioning Yeast

I have heard before that some breweries place a different yeast strain in their bottles to bottle condition their beers than what was in their primary fermentation. A topic recently came up on Brewboard asking about what kind of lager yeast to bottle condition a hefeweizen with.

It strikes me odd that a German hefeweizen would get bottle conditioned with a lager yeast for two reasons.

1. A lager yeast will not mask/hide what the primary strain of the original fermentation yeast was.
2. Don't lager yeasts need cool temperatures to ferment (i.e. do they work at warmer ambient temperatures)?

It is possible that the hefeweizens are kept in cool storage to bottle condition, but it all seems kind of pointless to me. The Weihenstephan 68 strain is perfectly capable of bottle conditioning on its own... and if a German brewer uses some different strain (I think Erdinger does), then I might be able to understand.

The one possible explaination that I can think of is to not get as much yeast trub on the bottom of the bottle. But, with hefes, you usually consume this yeast anyhow...