After re-reading the manual https://wiki.mikrotik.com/wiki/Manual:Wireless_FAQ#What_TX-power_values_can_I_use.3F and realizing I’m with Atheros 9000+ chipset it’s clear why I can use only all-rates-fixed mode.
After reading this thread http://forum.mikrotik.com/t/antenna-gain-missing-in-6-46-8/145162/1 I also don’t worry about using this mode.
One more thing — changing antenna gain from the UI does not seem to affect anything, as stated in the same thread.