Tosafos asks why we don't mention that Kalev davened improperly bec. his tefillah could have caused his daughter to marry a mamzer, yet received Asniel ben Kenaz. Tosafos answers that it was considered proper since he knew that his zechus combined with that of his daughter's would entitle her to a proper shidduch. Why don't we use the same logic by Avrohom - the gemara should not have listed Avrohom's tefillah as improper since his zechus combined with Yitzchok would certainly entitle Yitzchok to a proper shidduch?
Perhaps zechus helps to prevent spiritual deficiencies, but would not prevent physical deficiencies, since that is not as important. Therefore, it is true that by Yitzchok the zechus would prevent spiritual deficiencies such as pesulei yuchsin like mamzer, but the tefillah of eliezer was still improper bec. it was open to physical deficiencies. It is very meduyak in the gemara by yitzchok where it switches to "chigeres or sumah" rather than "mamzeres or eved" - since it was clear that zechus would protect against "mamzeres or eved" (this would answer tosafos question in d.h. yachol). But by kalev, there was no concern for a physical deficiency bec. the person would be a warrior it top physical shape, and since zechus protects from spiritual problems, all concerns were alleviated. That is why the tefillah of Kalev was considered to be proper.
