In addition, if you can prove that the debt is invalid (if you were not receiving the service you've paid for and made reasonable attempts to resolve it beforehand) then the collectors themselves will make a loss which should over time sour their relationship with the original company.
In my case, debt collections was the only way to actually get my problem resolved in the end. They've passed the info back to the company and they've finally cancelled the contract (and the "debt"), something I wasn't able to do with the company directly because I couldn't get through anyone competent enough to understand the situation and willing to fix it.
Credit history is a valid point however in my case I do not need credit anytime soon (the only potential reason I would need it is a mortgage in the far future) and in my country credit report entries expire after 6 years so it was not a problem.
Every scummy telco that makes it impossible to get in touch will suddenly try to call you and even work within your terms (in a normal situation, asking them to call back at a specific time tomorrow is a no-no, but suddenly when they're out of money they'll be more than happy to do so) once they need something from you. As a bonus you will typically speak to a competent person instead of a monkey.
Of course you need to make sure you're legally in the clear. In general, if they're not delivering the service promised and reasonable efforts to resolve the situation amicably were unsuccessful then it's fair game considering if they want to take it to the courts they'd need to first explain and justify why they weren't receptive to those efforts.