Do you pre-filter clients at all to ensure they have the budget? I've done scoping work for free, only to learn afterwards that the clients budget was 10% of what it needed to be. Do you just accept that as part of doing business, or is there a way to avoid it?
The question I put to you is, would you rather get good at extracting nickels and dimes from clients of a wide spectrum of quality, or would you rather get good at making sure the clients in your pipeline are mostly all high-quality? Billing for scoping time is a good way to get good at the former, and a good way to stay bad at the latter.