Recently, whilst dining in France I have noticed a custom that seems strange to me.
Typically in the UK I will expect a clean knife and fork for my main course after finishing my starter. However, on multiple occasions in French restaurants I have been handed back my knife and fork after having my starter dishes removed.
If this is indeed customary, is there a rationale for it? I can't imagine that it's done to reduce the amount of washing up to be done.
It's definitely not a French custom. Whilst cutlery has its own chapter in the dining etiquette, several restaurants do not replace used cutlery between dishes. Indeed, I have had this happen in various restaurants around the globe. The common denominator across all these establishment was their affordability. Keeping the same set of cutlery for more than a dish is indeed customary of low/mid-budget restaurants. And as you guessed, the rationale behind this is reducing the amount of washing up to be done.
In general, high-end restaurants tend to have all the cutlery you will ever need, neatly laid out on the table even before you sit down. There are cases in which specialised tools such as steak knives, fish cutlery, soup spoons, etc will be brought to the table and exchanged with the prepositioned cutlery. In all such settings you will most definitely be using different pieces of silverware for different dishes. On the other hand, low/mid-budget places will either bring the cutlery with the dish, or will have only one set of cutlery, consisting of a knife and a fork, laid out on the table.
You could just as well turn the question around: What's the rationale for bothering with several sets of knifes and forks? Minutes traces of food surely aren't a big problem and it does involve quite a lot of work, not only for cleaning but also for the service staff. One way or the other, such things are necessarily customary and this only strikes you as peculiar because you are used to something else.
That said, most restaurants in France will in fact provide a clean knife and fork and fancy restaurants will have several sets of utensils and a plate already on the table when you arrive. You start with the outermost set and then move on to the next one. The cutlery for the main dish are “stand-ins”, the staff will replace them with the correct ones depending on what you ordered shortly before bringing the food. Similarly the plate will be removed and replaced by the actual plate containing the food.
Simpler restaurants will have no plate and a single set of knife/fork and only bring special utensils like a meat knife as needed (possibly even putting it on the plate itself, e.g. with the blade under a steak to hold it in place). I have seen that in cheap café-restaurants, possibly some brasseries or chain restaurants. It does not bother me but I would not say it's common in France.
This is common practice in Spanish restaurants. The more upmarket types will replace your cutlery after each course, but most mid-priced to lower-cost eateries will expect you to use the same knife and fork for your first (paella!!) course and the fish and chips or whatever you have for the second course. I initially found this curious after moving to Spain, but soon got used to it.
NEVER. Unless you are trying to be be a (very) casual restaurant. If you have taken the cutlery up (either on the plate or it was handed to you), you always replace the cutlery with a clean set, appropriate for the next course. However, if it is not a formal restaurant and the client/ guest wants to use the same for the next course, no problem.
