From my experiene in real life, the alternate is more for radio failure than anything else. Apart from that, its just another piece of information that is never looked at. I for one work a fair bit of traffic in london and whenever something up, i am always asking intentions and alternate because we dont have full flight plan info. An example is all EGLL i/bs filing EGSS as an alternate......it is accepted but guaranteed they will oly take 6-10 divs only.
When theres something wrong at EGLL, you can bet you will not get your alternate just because of space issues and handling issues.
From my experience, it is only helpful in radio failure so that we in ATC have an idea of what you might do but unlikely to get it if there is a issue just because of capacity. I know EGCC is nominated as an alternate for A380s but they only have a certain amount of slots so once run out, EHAM, EBBR or LFPG here we go......