WordPress is easy to install yes! From a cost effective point of view themes are cheap and you can have a website up and running in a matter of minutes yes! And yes, arguably the use of plugins make the website scalable but not secure.

Using a theme that has been developed for a specific industry probably uses a plethora of plugins – most of which are under developed or might have vulnerabilities unless upgraded regularly. Of course there is the theme lockdown senario! Which means that your if your theme is meant to be a Recruitment, Design Portfolio, Recipe, Newsfeed site etc… then that is what it was designed to do, it was not designed to service the ‘one man band’ nor the ‘company blog’

As far as branding and making your theme look like your current website is debatable :-/
Themes are generally difficult to navigate and come with a mammoth manual that your developer will have to decipher – as for cost effective, a company might have to outlay more expense in training just so the developer can understand how the theme works. It could be cheaper to pay for a bespoke website that does exactly what the company wants it to do.

Like I’ve already said, I agree with what you are saying in the context of your post but, it’s not a question of ‘Does Your Company Need a Blog?’ but, in my opinion it’s ‘I am a small business, should I blog or not?’

As small businesses generally make the local and in turn national and in turn global economy turn should we not be asking “does your company need local business?”