Pick any friday session from last year, take out Marussia and Caterham... the track would be less busy obviously. Take out RBR, Mclaren as well... even more empty, nothing more than that.
Put it this way, Ricciardo's car broke, if they were allowed to run a different engine on Friday what would have changed, his car broke and he couldn't run.
Which team would have been able to run more yesterday should they have unlimited engines... none. Riccy's engine went, it takes hours to replace an engine, they put in a new engine anyway he still didn't get out. Marussia, didn't get out, Mclaren had issues, new engines wouldn't have got them out any more, etc. Horner has a problem, an unreliable PU, and thus is trying to manipulate the situation, nothing more or less.
If they break, they break, they take a penalty and keep racing. It's as with all regulations, if you aim for 4 then they'll be borderline and probably go over but they'll design with that target. Thing is, if they aim for 10 engines... they'll design with 10 in mind and they'll still go 1 or 2 over, that is how engineering works. If the limit is 10, they'll build something that can last 2 races but on the limit. If they had a 10 engines per year limit they would be FAR less reliable than they are today, they absolutely would not make the same engine and use only 5-6 engines as last year and get no where near the limit.