"My application doesn't need to be modular" means several things:
1) My application will never change, or will change so slowly that I will have plenty of time to make any and all changes.
Yeah right. Been coding long? Guess not.
Rapid change is accomodated by modularity - have it and you will be able to handle anything they throw at you.....neglect it, and you'll be neck deep in spaghetti with a red-faced manager screaming at you.
2) My application will never grow
Similar response as to number one - though this may not be quite as obvious to the noob - apps ALWAYS grow. Period. Unless the app just totally bites to begin with - system begets system. Whenever users actually get to use a system, it inspires them to want more system. Don't know how else to say it - but, if you build it, it will grow.
And if you build it monolithically - you're going to have to be reevaluating the entire app everytime you need to grow...not fun, and it definitely doesn't make you look like a rock star.
3) My application will never be maintained by anyone but me
Really? Nice. So you will NEVER leave this job. You will NEVER grow disinterested in maintaining the same code base year after year after year.... I don't know about you - but I'm usually bored immediately after it goes to production.....
The new maintainers will not be happy people if your code is a big monolithic mess.
4) My application will never be used for anything except what it was originally intended for
This one is funny - it's amazing how expectations change over the course of an application's lifetime. I wrote a system once that basically assumed (based on the existing business process) that only one person would be entering information at a time - and that the information would then be sent out and tracked to several hundred people. Unfortunately for the system - the organization grew by leaps and bounds - to the point that the business process started to parallelize - meaning more than one person would be entering information. Uh oh.
Because a user doesn't have your unique vision about the system, they will often use it in ways you never even thought of, let alone intended. If your application is not modular - you won't be able to accomodate this.