Regarding the Shinra Building:
Regarding the new boss (whose tail I loved): I had to make 23 steal attempts, with a character thats level 16, to successfully steal the item from the level 12 Grunt. Its kind of absurd how low the success rate of steal was in this case. Another thing was how useless magic was in the battle. A Bolt dealt 86 damage. A regular attack 49. A Ice or Fire dealt 57... Even if the thing is not weak to Fire or Ice it should have dealt more than that. Otherwise it was like I said before and magic becomes rather useless. Its good when the monsters are weak to it, but otherwise its better off to whack it in the head with a blunt object, and characters like Aeris and Caith Sith, who are more magic oriented, become less useful.
Yeah I do agree the steal rates are pretty bad. Crazy bad if you're doing a low level or no exp run. But I think magic becomes really important once your past Midgar, especially when they reach their next tiers. I'm doing a no exp run, and Cloud, combined with earring and four slots, can do 700-900 damage with fire 2

. You'll notice that magic will be amped and useful when they level.
I've been using magic pretty much the entire game, and started relying a little bit more on physical attacks when I started getting late game weapons,ultimate weapons, etc.
Some good combos I like: Mp Turbo + Earth (3rd tier can do 6k-9999), and Comet + MP Turbo (capable of doing 10k+ ). I also use regen and time, every boss battle. Just depends on your play style: if you want to focus more on melee, you'll have to rely strength+ equipment to decent damage. Another thing about magic, spells like sleep, mini, confuse, etc have 100% chance to hit.
About summoning.. don't have a solid opinion on it because I haven't been using it. I think I'll start using it more with my no exp run because they are capable of doing negative status effects. Down side is they can only be used once per battle.