As far as I know there aren't really any major upsides to Lv 7 mortar, seeing as Lv 6 can OHK max archers. But of course every bit of extra damage counts.
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Don't listen to the tards telling you mortar first at th9, that's horrible advice.
Mortars are key to your defense and if you take one out to upgrade it, you need stuff that can back the rest of your base up now that you're -1 mortar down. That is why you upgrade all your cannons first. Then mortars.
Personally at TH9 (maxed and doing legos currently) I did:
Cannons first
Mortars 2nd
Teslas 3rd
Archer Towers 4th (strong AT's for when I do AD's next and have lacking AD available)
Air Defense towers
Wizard towers (most expensive last).