A) Duels. Any critical and/or all but minor jock damage may save or ruin the day, so weapons like BRG, HMG and NB rule the field over any conceivable amount of armor that can be installed into those titans. A good DCS or just pure luck might save the day here. Or a couple of head hits in a row.
As a side note, to keep the duels interesting, one should try to check the suitability of both titans to the scenario. A single support is almost doomed to lose, or a titan with Heat Reg 1 isn't going to do much good in Hell scenario. A couple of restarts shouldn't be too tough for anyone, right?
B) Small team battles. IMHO, this is where the most exotic designs have a fair chance of pulling their trick. Enemy fire isn't yet too strong for armors to hold pretty good, so adding a bit might just make your team survive those few extra seconds to score a win. Special effect weapons and their counters (Shields, DCS, overdone heat regs etc) still have strong influence on the battle outcome.
C) Medium team battles. This covers most scenarios in which you start to think about using dedicated supports. The effect of downing a single enemy titan isn't as noticeable anymore, and the reliability of your weapons starts to matter more than their special effects, smoke & steam screens excluded, of course. Rather than trying to make the titans balanced overall, the designer needs to keep in mind the added synergies that the different kinds of titans have together as opposed to working alone.
D) Large team battles. You're VERY hard-pressed to bring down enemy titans ASAP before they down yours. No time to wait for a critical hit in heat reg to take its effect, nor to repair one, for example. Enemy fire is so strong that you're actually much better off doing sacrifices in your armor in favor of added firepower, as there's no way your armors can hold it for more than a few brief moments. Scr3w all the fancy specials, you need to consider how much damage you can do to your enemy, as fast as possible, as reliably as possible. (Does that spell cannons/missiles or what?)
On the other hand, as the number of enemies rises, there's usually at least some targets for any set of weaponry. For example, energy titans should attack shieldless titans and cannon/missile titans should take care of those with a shield. This also means that all sort of defenses are rather moot unless you can defend against anything, which you can't. Use that saved space to pack more firepower to kill, kill and kill some more. A very strong alphastrike capability is more valuable than its weight in gold in many cases, even if your long-term firepower isn't as great.