Do you know how few DMs don't allow them either?
Trust me, unless custom hybrids are somehow mechanically inferior than normal species, they WILL become the default option. And it won't be because players have an interesting idea for a mixed character, it will be because X and Y traits produce the best competitive edge. The charops will find the best mixed lineages and they will become the primary choices for PCs (outside of newbies and hardcore RPers). Why play an elf when you can be a hybrid elf/tieflng and some get the advantages of both?