Techmarines will no doubt be a central feature in many Legion builds - especially those like the Iron Hands where such characters are central to the ethos of the Legion. But overall, I suspect that the techmarines are a poor choice for a valuable elites slot if one is building a themed army that won't be "unbound".
One can purchase up to three techmarines as a single Force Organization Chart slot, and each of them act as their own independent unit. In addition they can be accompanied by servitors which serve in a number of capacities: ranged weapons, close combat weapons, or just general meat shields. And, of course, the techmarines themselves act to help vehicles survive on the battlefield. Ideally they'd be used around superheavy tanks, or at the very least those tanks that are going to be responsible for pumping out a good deal of firepower. Keeping those kinds of tanks active can be game changing, particularly if a weapon destroyed result gets reversed.
Here's the catch. They only do that on a 5+ roll. So what is the probability of that occurring? Well for 1 techmarine it is clearly 2 in 6, or 0.33. For 2 techmarines, it is 0.56. For 3 techmarines, a probability of 0.71. Therefore if the battlefield role is to keep tanks going, take 3 techmarines!
Below are a few builds to consider.
1 Legion Techmarine, 4 Servo-automata, conversion beamer (128 points)
A sort of cheap build. This one is meant to hang around a critical tank and keep it going. In turns that this doesn't apply, use the conversion beamer to lay down some solid firepower. And use the servitors as simple meat shields for the techmarine. A risk as well since there is only 1 techmarine. Add an augury scanner to taste.
3 Legion Techmarines, each with 4 Servo-automata, conversion beamer (384 points)
As above, but for when you absolutely must keep your shadowsword tank (or similar) alive!
3 Legion Techmarines (135 points)
The naked version of the above. Used purely to keep that vital tank alive. Good luck!
3 Legion Techmarines, Rad grenades, Thunder Hammer, Nuncio Vox (240 points)
Ride around in a transport. Aid another unit disembarking nearby?
1 Techmarine, Rad grenades, Thunder Hammer, Melta bombs, 4 Servo-automata with lascutters (143 points)
A sort-of close combat tank hunting techmarine unit? Better options than this realistically I think, but just pointing it out for completeness.