I just remembered a big reason why Vaaish is right. When a terminator or space marine gets trapped in a sticky situation, they have the advantage of being able to be teleported (at some risk) back to their ship.
Tau do not have this technology.
However, I am making this clear: The Taros campaign book is clear, the Tau Manta IS a military vessel, used for rapid and MASSIVE troop movements, as well as modified for space combat. Also, it has the ability for limited FTL... It would NOT be wise to compare it to, say, a Thunderhawk.
Considering that it also describes in the section about it's two massive railguns that it's sub-munition round is used against larger ships, the drone controlled ordnance finding weaknesses in the enemies armor, so blowing a hole to dump troops into should not be an issue, it's also why i added the possibility for them to fail (costing the parent ship one of it's mantas)