The vast majority? Says who? Overstayers and criminals can be redocumented and removed relatively easily once found (with a handful of exceptions depending on nationality).It's impossible to remove the vast majority of undocumented migrants anyway. Nor would we really want to.
Opening up the borders in Ceuta and Melilla would make it possible to control/direct the flow of migration throughout Europe rather than creating bottlenecks which bear the brunt of relatively small-scale inward migration. It would also stop dangerous sea-crossings. Meanwhile you could co-operate with Morocco to, rather than crack down on migrants (which drives people over the fences/across the sea) to be able to accommodate more of them.
And surely whether we'd want to depends on the particular migrant in question?
This is the nub of it really. Who is it you anticipate coming through Ceuta and Melilla? Can anyone come across and chance their arm at an asylum claim? If that is the rule, do you not anticipate a very significant rise in the numbers coming, irrespective of the fact large numbers will not be in any perilous danger at home?