The LTE channel spacing is fixed at 15 kHz and cannot be changed. The choice is pretty arbitrary, but it corresponds to a symbol time of 67 microseconds (1/15k), which is a convenient time interval for symbol processing.
MBMS transmission in LTE shall use either dedicated carrier or shared one. Subcarrier spacing 15 KHz is used where carrier is shared between generic LTE (Unicast) and MBMS. However the 7.5 kHz subcarrier spacing is used for Multimedia Broadcast Multicast Service (MBMS) transmission in dedicated mode where dedicated carrier is used for MBMS. In the case of 7.5 kHz subcarrier spacing , one RB consists of 24 subcarriers for a duration of one slot. A larger cyclic prefix of 17 microseconds in present releases and 33 micro sec (CP =1024 Ts where Ts = 1/(2048*1500) seconds) in future releases is planned to address the larger delay spreads that are likely to occur due to processing of the broadcast signals from multiple eNBs. Since a higher CP duration could imply a higher overhead, a larger OFDM symbol period was chosen for MBMS transmission leading to the smaller subcarrier spacing as the two quantities are inverses of one another.
This below link has some more information: