It may have some thing to do with the marketing and/ or business model for the given software in question but also one of making money from the sale of that software in the long term to recover the costs of development; it seems that there are four basic models used in the industry today, please baer in mind that is an overview:
Model #1: Time Limited Model (Then becomes Nagware, Crippleware or the like):
In this model the user is given a limited time to use the software normally 30 days but I have seen 90 day trials with/ or with out feature restrictions such as save disabled with the user reminded that they have X number of days remaining in their trial period. After this time any or all of the following occurrences may happen:
1. The software becomes Nagware and uses a pop up or other such device while the user is working that their trial is over and that they need to go and buy the software.
2. The software become crippleware where either or:
a. The software's features become further limited
b. The software will not execute on opening and the user is face with a splash screen telling them they need to buy the software if they wish to continue the use thereof.
Model 2: Feature Limited with or without use time limitation:
In this model the software is generally open for use (with some industry exceptions) without time limitations of the first model been save disabled is common, other restrictions in other software apply Silo 3D is limited in the amount of geometry (number of faces) that can be saved in its demo version, by way of example. Apart form these small restrictions users may use these demos for as long as they wish thus allowing for longer term evaluation of the software, useful for software that would take time to become fully proficient in.
Model 3: Water Marked Image (Water Mark Removed in Licensed Version) other restrictions generally do not apply (not in all cases):
In this model there are generally no restrictions on either time or features within the software and save is by and large enabled however the software generally automatically applies a watermark to an image in multiple locations on the image (Some just place a single watermark in the center of the saved image) this watermark is not present in the licensed software.
Model 4: Unrestricted Use (Non-Commercial Use Only):
In rear instances you will get a demo of a software product that dose not have any restrictions except that of been for non commercial use only, this aids long term evaluation of the software that other models do not encourage.
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There are many thorny issues with all of the above and have been since the early days of computing I remember that these kinds of issues where around with the release of both the BBC Micro, The ZX Spectrum and the Commodore Vic-20 and maybe before, but that would be before my time in that case.
Regards to you.
Cyber-Angel