How many people travel from Urmston to Wigan everyday though?
Rail is a mass transit system, it can't cater to every demand out there, so its better to concentrate on the busiest corridors.
Having a system where several places have alternative city centre termini (or stations) is just going to confuse people. Its a bit of fun for us enthusiasts to work out such permutations, but it just makes the railways too complicated for others.....
The point of the Urmston example was to get you to realise that not everyone goes to/from Manchester and no further, a point you have clearly missed, I can't quote exact numbers for any particular example, but I know people do travel across or via Manchester on a regular basis.
Of course you can't please everyone, but look at the logic.
The people it does not currently cater for are those who insist on a concentrated more frequent service from one of the stations.
The people it does currently cater for are those who travel further afield, those who want a choice of stations to arrive in, for those who really don't care where they get to in Manchester just as long as they get there, those who want a direct train to Victoria (regardless of frequency) and those who want a direct train to Piccadilly/Oxford Road (regardless of frequency).
If services were concentrated on one station then you would alienate at least two of the groups currently catered for in order to appease the one group not currently catered for.
To decide if it is a good idea or not you would have to look at numbers and we don't have them, but I can't see how it would benefit more people than it would adversely effect.