Office Communications Server 2007 R2 was the first version of OCS to offer PSTN (or dial-in) conferencing as part of the base product offering. It provided a relatively well-rounded set of features, especially when the presenter ran the dial-in conference from the Communicator client. Booking audio conferences was a breeze when coupled with the Outlook Conferencing Add-In. Presenters could easily see who was in the conference and could mute unruly participants with the click of a mouse. However, the presenter experience was lacking when dialed in from a regular phone. There was no way to perform any of the usual functions via a touch-tone phone like roll calls or global mute, which limited its appeal in circumstances where Communicator wasn't a viable solution for running a dial-in conference.
Like it or not, but many companies have yet to jump on board the OCS bandwagon for a multitude of reasons. A side result of this is that many users simply do not have the headsets or other hardware necessary for a good computer-based telephony experience. Since their only real choice for hosting conferences is via a regular telephone, the lack of in-conference features in OCS R2 limited its appeal to companies who either were already using Enterprise Voice, or ones who took the time to invest in headsets so their users could host their audio conferences from Communicator. This leaves a lot of companies with one less compelling reason to deploy OCS.
Dial-in conferencing in Lync will become a significant driver for companies to make that initial foray into Microsoft's Unified Communications world. The biggest reason is that the PSTN conferencing featureset has improved to the point that it can be a viable replacement for many corporation's existing hosted PSTN conferencing services. Presenters who dial in via telephone can use the keypad to perform almost all the features normally available via hosted teleconferences, which eliminates one of the key blockers for adoption.
Coupled with a direct connection to an external SIP provider such as Thinktel in Canada, companies can easily host their own dial-in conferences at a fraction of the cost of traditional hosted teleconferencing solutions. If a company is already deploying Lync for IM and internal conferencing, it's very easy to add the dial-in conferencing functionality, since the mediation server can be colocated on front-end servers and a direct SIP connection doesn't interfere with the company's existing PBX.
With this "foot in the door", companies can get a real feel for the stablility, ease-of-use and cost-effectiveness of Lync. Over time, companies may be more open to moving their main telephony solution to Lync and can start to take advantage of the entire featureset available.