Two Questions about Kitchen Wall Tile

RDS

Cement board flexes more than drywall, not less -- this is easily observed.

Thinset bonds better to cement board than to drywall, but if you use mastic instead of thinset this problem is moot.

Water infiltration is not a massive problem with a kitchen backsplash, and as Bud says there are things you can do to protect drywall from moisture if you are concerned about it.

Cement board is much more difficult and messy to work with than drywall.

Conclusion: Use drywall behind your backsplash tile.