Maintaining software is hard and therefore expensive; and IT departments are often under-funded. However, if they are in a Just do it mode, then non-functional requirements are easy to be forgotten. The consequences of leaving these NFRs lead directly to the aforementioned maintenance problems and increased technical debt.
NFRs are necessary to finishing the IT applications journey. While one might consider two or three important NFRs (such as performance and security), one will probably not cover the others extensively, or you might miss out on them all together. And if one does allocate time to deal with them, when the schedule slips, the NFRs may be the first thing to get dropped. So, whether you plan for NFRs or not, chances are high you won't cover them 100%. One should try to avoid adding technical debt and maintenance nightmares to the application portfolio.