Why pick just a single bacterial transformed colony
This is a matter of pragmatism in the culture process. 

Taking 100 colonies instead of 1 increases the inoculation volume by a factor of 100, which then saves you perhaps 2 hours of bacterial growth time before your culture reaches the OD you want. 

However, mutations and loss of plasmid in culture, while unlikely, are possible, especially if the bacteria were not cloning strains with their recombinases knocked out. In such a case, you would risk the downstream experiments being contaminated with mutants, be that protein expression, retroviral plasmid production, or something else. 

Worse still, the mixture of bacteria containing mutants is less likely to be detected, since had you picked a single mutant colony, there would be absolutely no result, but a contaminated culture would produce a low but still positive result. Therefore, you may end up wasting many days or weeks troubleshooting your downstream experiments.

Therefore, picking single colonies is simply a matter of "better safe than sorry".