I agree not only do you give up armor but you give up those nice shields too and my number one rule for creating spellcasters is:
NEVER MC you main spellcasters since every single level you take in another class takes away from your highest level spells.
The ONLY exception to this might be if you are playing HOF and then you could consider adding a second class after level 20ish as spells get pretty few and far between then.
As for clerics think two or three times before choosing an evil cleric as well because spontaneous casting of healing spells rocks while spontaneous casting of harm pretty much sux...
Think hard before choosing an ECL race as well becuase each level you lose is more high level spells you miss out on.
If you want a melee cleric consider Tempus especially if you don't have an axe weilder picked out yet but most any cleric can tank pretty well when buffed up if you build them well.
Persoanlly I like the Lorekeeper - decent domain spells and free ID once a day (even if you don't choose the ID domain spell.