OK ... at one level you're right, a single bomber flight can remove fighters from a field just as a single fighter can remove troops and bombers from the same field.
But there are two gaping differences.
One is the time investment. Popping a Tiffy to 10K, scooting over to the enema base, and then porking bar/ord takes a lot less time than hauling Lancs or B17's up to some reasonable altitude and then lumbering along to target. So there's one big difference - time investment and, as a result, frequency of occurence.
Then there's another big difference - rate of success. A Tiffy Porker at 12K is virtually unstoppable if he gets within radar range of the base. Partly because its a fast plane, partly because it takes so little skill to succeed with the 1.0 pass needed to pork bar or ord, and partly because the porker doesn't give a damn if he dies (and certainly doesn't care about fighting anyone until after the deed is done).
A flight of heavies at anything under 5K stands a much smaller chance of success. Granted, those B17's or Lancs down low may gobble up some figters in their defensive fire, but odds of them getting to drop are much, much lower than the high speed porker. (Not that I'm advocating NOE heavies, mind you.) And if the heavies are at altitude their odds of a direct hit on all the FH's is far from that of the odds of a porker hitting the bar/ord.
And there's a third difference. A flight of heavies can affect one base - maybe two if they don't get intercepted. A single Tiffy can remove troops and ord from 3 or 4 or more fields - and probably in less time that it'd take a B17 to climb to 15K.
And while I'm at it ... a fourth difference. Most of the time I see heavies attacking a base as part of a capture attempt. Porking is a defensive tactic (except when protecting a CV from coastal fields). A couple flights of B24's can make a capture attempt at *one field* hard to stop, a couple of Tiffy Porkers can make any capture attempts along an entire front impossible to accomplish because of the game mechanics of the host.
So, in terms of gameplay, we are really talking about two completely different things, even though they end up resolving to a common theme.