...after a recent experience I will never again let a designer hold me hostage by not turning over the PSD...
There are legitimate business reasons to do these things but if you want to avoid the situation, make sure to have it clearly stated before the project begins that you expect to have full rights to the source files. Even better, include it in the contract. Or have it written up clearly and agreed to in an email before the project starts.
And if you ever have a wedding, the same thing applies for your wedding video or wedding photos. Work it out ahead of time that you will actually own the raw video or photo files.
And don't be surprised if the prices that you're quoted go up when you ask for these things. Often, contractors will quote low prices knowing that they will make more money in the long run from followup work. It's not wrong for contractors to do that, but it feels dishonest when it's not clearly explained ahead of time exactly what you're getting when you pay for contract work.
A software company that won't turn over the source is going to resell/reuse that code. Your analogy implies that a design firm will reuse/resell the PSD and it's hardly a reusable item.
For example, in photography, the photographer keeps the negatives (or .NEF files these days). What you get is the finished product - colour balanced, retouched, cropped, and probably actually only 5-10% of what was shot. That's what you're paying for, the final images. If you're the bride, then the photographer's business model is to give you a good price on shooting your wedding and make money on reprints. If you're an ad agency, the photographer licenses you the image for this campaign for this much time, because the business model is that that image will end up in a stock portfolio in a few years.
If you get the originals, and exclusive rights in perpetuity, then expect to pay a premium for it. It's up to you if you actually need them.
I think this might be more common for flash stuff and the designer keeping the fla and giving the swf away(obviously).
Next time you should clearly state that you want the psd's at the end, but be aware you most likely have to pay more, because the person is giving you the source.
For the record, I think that the standard wedding photographer scheme is sleazy.
My data belongs to me.