great question. While the rule will work either way I agree with you that it makes high-action targets extremely hard to hit, especially if they are using cover and such. Just remember that a 6 is always a success regardless of the DC.
In my games I consider combat rolls above the DC as successes (personal preference as it can speed up combat). But you can just as easily keep the defence DC as requiring an equal number of successes to hit. This isn't as limiting as it might at first appear - and balances for all considerations.
Take for example the tanarii Alchta (soldier) pg122, they would be a fairly standard encounter for relatively inexperienced characters. With an action 3, a character would need to roll 3+ on 3 dice to successfully hit. On average most characters will roll 5 or more dice on an attack roll, then gain bonuses from modifiers, gear, etc. So you can see it's not too tricky, but enough to give a good level of challenge. It also clearly highlights the benefits of good cover, good training (soldier vocation, combat talents, taking time to aim) and good gear (scope, targeter, etc).
Also don't forget that someone with 6 Action score moves with lightning speed and preternatural agility (think Neo in the Matrix). The DC chart describes success against a DC5 as jaw-dropping, so succeeding against DC6 is phenomenal.
Hope this helps.