With a 3.5 alienist, you can't summon any non-templated creatures. You are restricted to summoning creatures that already have a celestial or fiendish template, which is then replaced by the pseudonatural template.
That said, assuming you permited it to be applied to non-templated creatures, it's a relatively insignificant boost to the power of the creatures in my experience. The only substantial gain in power is the true strike and that's only 1/day. The ER and DR are nice, but rarely make that much of a difference given the low HP and AC of most summoned creatures. Personally, I wouldn't mind simply letting it be applied to any creature on the summoning list. It just doesn't make that much of a difference given how weak summoned creatures are.