Take Mont'ka when you want to play proactively and decide the game in the first half, and when you are content to field a single detachment rather than a combination. At 3 DP it is a complete army, and there is a real advantage in that: you get six stratagems and four enhancements all pointing the same direction, rather than two half-suites that do not talk to each other.
Be clear-eyed about the two costs. No partner detachment, so no 1-DP cadre patching a weakness. And no detachment rule from the fourth battle round, which is why Exemplar of the Mont'ka at 10 points is better value than its price suggests.
Sample gameplan: deploy to threaten two objectives rather than one, and use Strategic Conqueror on the objective you intend to hold rather than the one you intend to take. In round one, advance everything, because [ASSAULT] is free, and use Aggressive Mobility on the unit whose Advance roll you cannot afford to gamble on. In each Shooting phase, spot the single unit that most threatens your plan with your cheapest Markerlight Observer and put all your Guided fire into it. Spend Focused Fire in round two when two units can share a target worth killing. Keep 2CP back every turn for Counterfire Defence Systems. By the end of round three you should be holding more of the board than your opponent, because from round four you are a normal T'au army with an expensive enhancement.
If you would rather have a partner detachment, Kauyon at 2 DP is the aggressive player's compromise: it plays the opposite half of the game but leaves a Detachment Point to spend. For the fundamentals, read the T'au Empire army guide.