Basing more planes before capacity increases are in place would be folly. Instead MAN needs more away based planes flying in at those non-peak times to fill the gaps as Ryanair currently do.
Some would argue that turning away the prospect of additional based units is the real folly. Many companies will be unwilling to instead deploy away-based aircraft at the bidding of any one airport operator. Airlines have plenty of attractive alternative options for deploying their fleet resources.
There is a dilemma here. An airport operator is not in control of what new business individual airlines are prepared to commit to its facility. If an airline such as Norwegian applies to base six aircraft at MAN (as has been rumoured recently) it is incumbent upon the airport to ensure that they are in a position to accept such business without hesitation. If they turn down the opportunity, they cannot presume that the carrier concerned will come running back to them when new terminal / apron capacity comes on stream at some point in the future. Worse still, that carrier may instead establish a successful operation at an alternative airport which represents direct competition. Once the carrier is embedded at the competing airport, it can eat away at market share for years to come.
Before turning away new based units it is also important to keep the maths in mind. Whilst most readers here celebrate the tremendous boost to MAN's status represented by the new 4x weekly Hainan Airlines service, an additional based Ryanair B738 working a full schedule offers nearly four times as many seats through the airport in the course of a week. Such opportunities should not be turned down lightly. The offer may not come around again, and that aircraft could end up based at LPL or LBA instead. Great for those airports, yes, but MAG must be focused on maximising their own business.
In reality, it may not be physically possible to accommodate all applications for based aircraft at MAN. But if the airport does have to turn business away, that represents a long-term strategic failure. MAG does need to redouble its efforts to provide sufficient stand availability at MAN to meet demand from carriers in the near term. And those stands can't be just anywhere either ... a Ryanair B738 requires T3 access, not a space built over 'Staff West' ... so careful planning and commitment of financial resources is required to address the problem.
Of course, one factor which may come to MAN's aid on this may take the form of the next recession. Not something we want to see, but something that we inevitably will see. When that time comes, some of the weaker operations will be stripped from the market (as always happens) freeing up capacity for the more resilient carriers.
Slightly-related question: will this latest proposal to link Pier 4 with the 1974 root of T1 Pier C represent an overall increase / decrease in aircraft stands vis-a-vis the previous incarnation of the plan? Presumably this arrangement implies that the new pier would be one-sided with aircraft able to access only the West / South aspect of the structure?