Of course specifics and circumstances matters. If a tool is critical for the business then we will review everything in depth and pick the best, regardless of trial/free plan/etc. And of course, sometimes the critical task is simply to ship ASAP...
My experience is that I rarely have enough time to determine if I really like the service or if it's still just a shiny new tool to play with. I need more than 14-30 days to actually put in the work to understand a reasonably complex tool (say a CRM system) well enough to make a commercial recommendation.
Obviously this issue doesn't apply to all SaaS companies but it's probably common enough to be worthy of consideration.
I, personally, rarely use a trial period for anything I’m testing for the long term. It’s not worth integrating it into my day to day if I may have to pay more then I want.
BUT, I do think it’s important to offer a free level for a lot of simple services to get people hooked. Costco does it, and it works for a lot of software.