[Lazy_Perfectionist]

Smite only affects one unit. And check that table on page 5.

So, I'd avoid adding penetration to smiters, usually. Though if I had an area attack-mr spell, a 6-15% change in results can be handy. I might equip them for single target spells if I have some unit I need to take out with the units available- but I wouldn't dedicate it to a smiter, since smite is usually good enough for most small fry, and more smiters are much better than a small change in percentage.

Ctis:
Barring special occasions, I'd give the penetration items to my Sauromancers/Marshmasters/etc - units capable of casting shadow blast, or other similar spells. Lizard Kings will find penetration items useful, but in most circumstances your mages will make better use of it.