Little Apps or More Modules in the Core Product

Sometimes we get the urge to build a little app on the side to solve a certain problem that isn’t as core to our main product (like billing). Fight the desire, we must. Building a small app outside the core product introduces a host of challenges like:

• The fresh excitement to build an app from scratch wears off when no one wants to maintain it on a day-to-day basis
• Testing and QA won’t be as rigorous due to higher priorities
• Interface changes won’t be implemented consistently due to the separate code base
• Server monitoring and administration infrastructure won’t be as thorough due to unique aspects of the architecture that are different from the main app

My recommendation is to avoid making little apps when at all possible as maintenance of them becomes a serious challenge. Unfortunately, they won’t be given the necessary on-going attention.
What else? What other thoughts do you have on making little apps vs more modules in the core product?