Yes, the only good reason for having a combination oven is if you are short on space, and just don't have room for multiple appliances.
The saying "Jack of all trades, master of none" is just as applicable to devices as it is to people.
I disagree, we have a Panasonic combi microwave, fan oven and grill and it is very competent at all 3. The only limitation is the size of pot or tin that will fit in it but it's fine for us. Using the fan oven I do pies and lasagne regularly, also casseroles and roast birds. Just not massive turkeys etc. Cheese on toast is spot on using the grill. Better than the regular grill as it turns the food so grills more evenly.
We also have a double conventional oven which I've not used for years as the combi does it all so well.

