Exactly - if you trademark something but do no trade, you can't demonstrate losses to a court if someone else uses your trademark. You also have to defend the trademark against infringement, too; a business can let two or three or four instances of infringement slide (maybe figuring it's too expensive to take to court), but if they take offender #5 to court, figuring "Enough, already", the court may well rule that the first four uncontested infringements constitute abandonment of the trademark.

Further, trademarks aren't all-encompassing; the idea behind a trademark is that customers should not readily confuse two businesses. Apple Computers uses the apple-with-a-bite logo for their brand of computers, but a company in the apple-harvesting business could probably use a similar (not identical) mark because of being in a completely different area of business. There's very little odds their customer base is likely to confuse their product logo with that of a company that makes and sells computers. Disclaimer: This is a layperson's understanding of the intent and application of trademark law, so take it with a large lump of salt and by all means consult a solicitor.