When comic strips first started to become widespread in the early 20th century, many parents and teachers attacked them on the grounds that the brief text in the speech bubbles and illustrations discouraged children from actually learning to read. Thus narrative texts were introduced in order to counter this criticism.
I'm sure you're right. I recall reading somewhere (possibly in Harry Thompson's biography of Hergé) that when Hergé's very first Tintin strips were being prepared for print in Le Petit Vingtieme, someone tried to add narrative prose beneath each panel, something that Hergé successfully fought against.
What's odd about Jacobs being so fond of his unnecessary prose panels is that he worked so closely with Hergé on some of the best Tintin books, clearly contributing much to the artistic and visual-storytelling qualities of the adventures. You'd have thought that something of Hergé's brilliant economy and tighness would have rubbed off on him! But maybe he wanted to find way of distinguishing his own stories stylistically from the Tintin adventures.
Your point about Jacob's prose being better written in the original French is very interesting, luinivierge, and may well explain why they jar quite so much to us UK readers. Certainly the B& M English translations (especiallly the most recent ones) often seem rather leaden, in both prose panels and speech balloons, sometimes reading like a laborious and inappropriate word-for-word translation from French, rather than being more freely translated into natural-sounding good English as the Tintin books are.
Personally, I find I can learn to accept and even enjoy the over written prose panels as part of Blake and Mortimer's odd charm, along with Jacob's affectionate but slightly strange portrayal of Britain (such as the fact that all the high-ranking establishment figures, including the heroes, read the Daily Mail!)
But I can never just lose myself in a Blake and Mortimer adventure the way I can with a Tintin book, and I think the way the prose panels constantly intrude, to remind you you're just reading a made-up story may have a lot to do with that.