Access points are as cheap as chips (so we can easily fill in dead spots), can be given plenty of bandwidth in the internal network and most importantly, we don't have to pay business mobile rates for internal calls (about 80% of mobile calls at work). The VoIP system sends incoming calls to both desktop phones and WLAN-connected mobiles - for free. If the mobile is out of range of the WLAN (i.e. out of office) then the system justs redirects to the mobile on GSM or to local voicemail.
The femtocells just make more money for the mobile operator and provide extra lock-in to their particular network.