Yes the N900 is open and you can install whatever you want on it, but you cannot compare PR1.2 with meego and mer: the first one comes from a relatively closed source where we don't know exactly what has been done (think modem firmware), whereas meego and mer are completely open developments, with people actually having the knowledge of the product.

As someone already stated, helping out people who flashed something not openly available is difficult and a huge time sink...time that could be used to improve Mer, meego, applications, etc.

And concerning modem firmware: it was a more or less known fact that PR1.1 flashed the modem and caused problems with downgrades, so it's not unreasonable to think that the same thing would happen with PR1.2.

And I don't even talk about the legality of flashing a leaked pre-version PR1.2...