The L2MTU value in RouterOS does not include the MAC addresses or the 2-byte EtherType/Size, nor the CRC. So 1514, which was for a few years (until recently) the default value for the RB5009 was well enough to allow 1500 IP MTU + 8-bytes PPPoE + 4-bytes VLAN header because 1500 + 8 + 4 = 1512 < 1514 for example. You can watch this MikroTik video for the details (from 3m02s):
https://www.youtube.com/watch?v=7a_z1jAdIME&t=182s
The downside (from that video above) is that the buffer consumption might double. However, in the recent RouterOS version the default L2MTU of the RB5009 has been increased to 1596, which means there is no memory usage difference between setting l2mtu=1514 and the default l2mtu=1596 anymore.