In past mergers (Delta with Western Airlines, Republic with Northwest, American with TWA), the airlines involved have promised that the "new" merged company would leave just as many choices for their passengers in terms of the number of flights between what the industry calls city pairs (Los Angeles to New York, for instance). But those promises, however honest the intent, are seldom realized. Northwest, for example, bought Republic Airlines in the '80s primarily to get its large hub operation in Phoenix, then promptly gutted its investment by abandoning Phoenix so completely that a struggling little startup airline called America West was able to survive and prosper. The war between the pilots of Northwest and Republic -- fanned into a frenzy by a misguided Northwest management using a divide-and-conquer strategy on the unions -- still festers to some degree to this day.
By contrast, Delta's acquisition of Western was handled very professionally, with Western's employees treated with a respect seldom shown the workers at acquired carriers.
Finally, don't worry about your frequent flier programs being gutted in a merger. No sane airline leader would consider such a move. Will they continue making it nearly impossible to redeem those miles? Of course. But dumping the programs, never.
Unfortunately, we don't get to vote on whether Delta falls to U.S. Airways-America West, but if it happens, passengers who pay close attention to the fine print and the changes will fare well.
Those who don't will end up standing all alone at the wrong counter as their flight pushes back somewhere past the security portal.