Yes they do it but I just question how viable a company is that relies on blocking ads to display their data.

If it drives more people to the online version of Gmail then that's still a win for Gmail in some sense (if not monetarily). I currently use Gmail via IMAP so never see any ads but Rapportive could tempt me back online.

Yep, especially if it gives them a lot of useful information about you to target ads to you in the rest of the googleverse.

Can't see it being a good thing when Google are deciding what company to buy though - would anyone buy adblock?

I absolutely hate plugins/companies that just routinely cover up other peoples advertising. It's not a smart move IMHO.