Two reasons that I can think of:

- Pidgin has a set of features based on long time development, whereas the integrated IM has a minimum of features and tries to reinvent the wheel. The buddy list in a multiprotocol IM client and the contact list in the N900 are two different beasts completely. At least today.

- People don't want their every world wide contact to show up on their list when they want to dial out. Also, they do not want every people on their phone list to be able to see their online status. For instance, I would not have the pizza place see my online status, nor would I want the support dude on my foreign web host to list on my carphone kit.

All this could be fixed by adding multiple contacts lists, better filters and more in the integrated IM client, and most importantly, a good buddy list. But as it is now, I don't really feel using the integrated IM client is good enough to use.

In principle, though, integrated IM client sounds like an excellent idea... But several hundred names to scroll through when in a hurry to dial out doesn't.