I don't think SD are aiming for any combination to be better than any others, just different. Horses for courses and all that. Be carefuly not to be too drawn in and think that a Heavy Engy is gonna be some uber death dealer. Engineer is meant to support the team, not be a one man army ala TF2. Heavy Soldier will have the greatest firepower most likely through class abilities and buffs, but this game will hopefully be a very team oriented one. Just try not to mistake the Engy class for something out of TF2. You make it sound like the equivalent of the HWG building Sentries whilst weilding a minigun and standing next to a level 3 sentry, which won't be the case (hopefully)
You point is very well taken.
I always play as a team-first player (which is why my Battlefield: Bad Company 2 "Squad Score" has been in the top 100 in the world (XBL), and remains in the 1,000.
But consider the "escort the 'bot" mission in the Container City Part 1 video on brinkthegame.com. Someone must be beside the bot for it to move, and engineers must stand ready to repair it. A Heavy-Engineer is a prime candidate for that job, walking with bot, keeping it repaired, and freeing everyone else to clear the way for the bot.
A Heavy-Engineer also has more ability-pips (or however the system will be structured) and so will be able to buff more teammates and is thus better able to help out the team in that way than a Light would.
Then, add the contents of my post above to the mix. In the proper circumstances -- death-zone. :gun: