Some thoughts:
Airlines like training bonds ( or similar ) but do not want to get involved in litigation.
I flew for an airline whose bond was complete fiction. It was ill thought out and there were at least three different break downs of 'costs' which had been put together by Chief Pilot, Finance Director etc, all were at complete variance to one another. Due to high turnover of pilots before the expiry of the three year period, the Company was very keen to be insist upon repayment of bonds. It reached a point where they did deals with some pilots in which costs were reduced in secret if the pilot told his mates that he had paid in full. Costs have to be reasonable, if not, a Court of Law could take a dim view. Balpa were itching for a fight with this particular outfit but as far as I know, nobody volunteered to be a test case.
It is my understanding that NOT signing a training bond will not make such a bond invalid per se because acceptance of a position and subsequent employment would be interpreted as acceptance of the terms and conditions offered.
Threatening letters from solicitors may not amount to much as they may be retained upon a ' no win no fee' basis.
As others have intimated, aviation can sometimes be a small world and a call from one CP to another CP could be as issue. That being said, if demand exceeds supply, a prospective employer may not pay much attention to the manner in which a pilot applicant leaves his current employer.
Food for thought, thats all !