Dont see why thats a problem. I mean if someone wants The Pacific on bluray, it will cost them around £70 or something. Not much difference imo.
I think you could argue the case there is a big difference between the 2 , but its about convenience as much as anything and how much thats worth to you. If your already subbed to Sky Movies, then its not much to worry about, but if your weighing up the pros and cons of subbing for 3 months to watch them ..
Blu-Ray - video / sound much higher bit rate + resolution difference, i.e Blu Ray @ 1080p
Blu Ray will probably have DTS MA / HD or Dolby True HD perhaps not for everyone, but anyone with surround kit especially that decodes HD formats would appreciate the difference.
Extras , documentaries etc.
A lovely shiny case taking up room on your shelves.
If you sub to Sky Movies just for this and Sky+ them, i believe once your sub runs out you wont be able to rewatch even its saved on your Sky Box Hard Drive. So you'll be in effect paying more to watch lower quality versions over a finite amount of time, perhaps only 3 months if thats all you've gone for.