I have one positive reason for a demo, at least for PC users - testing to see if your hardware can run it, when you’re unsure where your spec sits compared to minimum/required
I know most hardware is obvious in terms of how it shapes up, but some isn’t. I always like to see demos - I think it shows honesty and openness from the developer/publishers, if nothing else. However, they can be a pain to run for a number of reasons, so I do see why some games lack a demo completely.
Do you think a public demo/beta pre-release might of helped public opinion?
In today’s gaming market we normally expect some sort of demo being released for any new IP’s being released. With the current trend of gamers only purchasing sequels (due for many reasons you can read bout elsewhere on the net) it is almost mandatory for customer awareness of the title…ect.
I am aware that Splash Damage has stated that they have not released one due to time constraints but if they thought it would have helped sales they would have made time I ma sure.
The other problem, at least with MP games, is what to do with the “freebooters”. Since demos tend to bring in huge player numbers, they also bring the worst of player habits. When you have people who’ve just paid full price for a game, being paired with players who are simply trying out a free demo, it can ruin the fun of your paying customers. The last thing you need when you’re on the last objective in a really good match, is some idiot throwing grenades in their own team’s spawn area because they watched “sales guy vs. web dude” and thought it’d be funny 
So, this often means having “demo only” servers for the freebooters, to keep them contained until they’ve decided they’re going to pay, and be good players. This is extra expense - and you’re essentially entertaining people who might never consider buying the game, but are happy to try out a free demo with few constraints. Another potential negative, is that a good potential player hops on a server full of team-killing idiots, and decides the game is full of team-killing idiots - and therefore not worth the money.
I do think publishers get away with not supplying demos these days - it used to be a given with most big releases, but now it seems to have gone the opposite way. Mostly I presume, because a lot of big releases are utter mainstream crap and not worth the money 