Inkscape is nice and I use it all the time, but I am not a designer so it works for me just fine (I suspect is the same case in your office).
On the other hand, my sister is a designer, and there are so many missing features on Inkscape that prevent her to use it in her profession life. The comparison seems unfair, but it is the closest thing to an alternative for Illustrator in the software free world.
I find that many programmers find Inkscape or Gimp more than enough for their graphic needs, but it's really hard to come across a designer that would agree. Nobody likes to pay and pirate software; if designers found Gimp really up to the task, they would be rushing to use it and drop the Adobe monthly fee. The fact that this mass migration is not happening and has never happened in the past 10 years should tell us more than our own understanding of graphic tools would let us deduce.
I think that for a professional designer a £15 (or whatever it is now) monthly charge that can probably be expensed or tax offset anyway is small compared to learning a new UI or the potential difficulties that can arise when handing a non-Adobe file to a print shop.
What Adobe have achieved with photoshop is become the de facto standard for many aspects of design and image work. They have done this through a mixture of (1) being the best practical option and (2) some vendor lock in. We may not like it but well done to them.
Well, the limitations for GIMP in Print work is obvious and already mentioned above, but also consider the fact that "Photoshop" looks far better on a designers resume than being an expert in GIMP.
On the other hand, my sister is a designer, and there are so many missing features on Inkscape that prevent her to use it in her profession life. The comparison seems unfair, but it is the closest thing to an alternative for Illustrator in the software free world.