Small groups would have been suicide, strength in numbers. They were clearly out numbered by the TIEs, wouldn't take long for them to pick off small groups. Also would have been futile for them to turn around and engage the fighters, they would have endless scores of TIEs to deal with, they could never take them all out, it would just be a matter of time before each X-Wing bit the dust while wasting time looping around and chasing down and dodging TIEs.

The point was to get to the exhust port and shove some torpedoes down its throat. It wasn't that they were just letting themselves get picked off, they were fighting back, but they had to stay on their course or they would be going nowhere only to die in the process.

The stratedgy was very clear, get to the exhust port, stay low and avoid the turbo lasers, they knew it was likely they would have to deal with fighters, the stratedgy was evade, not engage. X-Wings and Y-Wings both have excellent shields, but when you are looking at one Rebel fighter for every ten or so Imperial fighters, there is just really no way to fight back. Evasion was the only hope to make it to the target intact.