If a person can't afford to tip properly, they can't afford to eat at that particular establishment.
Personally, I think we'd all be more generous, cheerful people if everyone had to spend at least a week in their lives waiting tables.
Also, if you tip poorly and return to the same establishment, expect to receive poor service from there on out. Your server will remember you and I can guarantee that they will pass along the information to any other server you may have. At the very least, the server will then devote their time to other tables at the expense of your service. At the worst...well...I'll leave that alone.
"Liberty lies in the hearts of men and women; when it dies there, no constitution, no law, no court can save it; no constitution, no law, no court can even do much to help it." - Judge Learned Hand