Firstly I must say, I have made good kit beer without temp control and have competition score sheets to verify it. Note that these beers were made in Aug - Sep in moderate to cool weather. The fermenter was at around 21 Deg.C and was at the mercy of the room temp. I have never made a good kit beer above 25Deg in my opinion, but I have drank them, since they were still OK - like they were not off.
However without temp control, I found that results were not predicatble and sometimes they were good and other times they were just OK.
With Temp control I started to use a wider variety of yeasts. I do agree that the kit yeasts are more forgiving for high temps - I let some wyeast brews get a bit warm and they were very bad. But if I get the temp right with the right yeast then what I get is better beer and more predicatble results all year round.
Temp Control for the puprose of fermentation is only one aspect of temp control I use. I keg, and also find say that for an ale if I ferment at 18 Deg.C until complete and then chill to seving temp and then add some Isinglass / finings for a day or so, I can drop alot of yeast / haze forming compounds. This creates a clearer beer and in my opion a cleaner tasting beer, and a little less "Kit" character. Then there are lager processes ect.
I find this works well for me.
Fear_n_Loath