Generally, I think our customers fall into two large buckets:
1. Experienced developers who want something lighter-weight than, e.g., Heroku.
2. Inexperienced developers (perhaps even first-time developers) who are trying to do something simple (e.g. send themselves a text message when a webhook fires).
To take your Stripe example, I imagine based on your question that you're thinking about the case where you have a server-side app already, in which case handling Stripe is very little extra work. If you are instead, for example, selling a digital good via a static website, Webscript would be a lot easier than building a full web app. You could have the job done via Webscript by the time you created a new app and set up your git repo for Heroku.
As to the DoS attack angle, do you have an example of how you see Webscript being used for something like that? If you mean flooding some target with HTTP requests, it seems like it would be more efficient for an attacker to send the requests themselves rather than try to funnel them through Webscript.