I play Zilean and don't buy boots as a standard part of my build, but I have to agree, boots are a must. As Dr Niles Aquatic said, mobility is everything! I pick up timewarp (haste/slow) as my second skill (and can spam it with rewind) so I usually don't need boots at all (end game I get 7 seconds of +65% movement off it, and can cast it right away after it's done, so it's basically permanent when out of combat) but if I'm laning against two champions who go boots early (before level 5 or so) I buy boots as well (not having them lowers my survivability immensely) but sell them later on when my mana regen can handle keeping my haste up. With no boots, I expect to be the fastest character in the game as a sprinter in the beginning (when no one else, except Rammus, has the skills to support it) and an endurance runner near the end (when Yi stacking speed and with his ult up can outdistance Zilean until his Ult dies).
Also, I do take Ghost as one of my summoner spells (and Flash, but previously I used Ignite for the +10AP during cooldown to go for pre-spawn kills). The speed boost from that replaces boots for me as boots will only make a difference once in awhile (when combined with my heavy use of Timewarp) and chances are that Ghost will be there to get me out of there or give me that little boost I need to catch the guy before he gets to his tower's cover.
As for your friend, you're right, Taric absolutely needs boots especially with him being a melee fighter. He isn't the fastest runner out there (although he's really close), and with the way his stun works increased movement will give you more influence over stun length vs dmg. Also, since he doesn't have a natural escape skill they are that much more important, even if he were using the same summoner spells as I'm currently using. Teleport is another spell that could be required for him if he has no boots, as trudging back to the battle from town must be slow going with no movement assists.
One last thing... If your friend is that opposed to boots (which is understandable, they are a waste of an item slot) he can use any of the other movement items (Zeal, Phantom Dancer, Force of Nature, Trinity Force, but not Lich Bane) and end up a tiny bit slower than most characters with the basic boots, while two of them together will almost equal or exceed most characters with upgraded boots (lvl 2).