Definitely. The logic should be able to accomodate a reasonable number of levels (maybe up to "cart contains x AND y AND not a AND not b"), but even a simple "if cart contains x AND not y THEN present offer z" would work wonders. There are a few scenarios, but it always boils down to offer a product, with or without a discount. We might offer a massive discount on cPanel if the customer is buying a $100 / month VPS, but a tiny discount if they're buying our $20 / month VPS.
If cart contains x, offer discount y on product z for # months
If cart contains x, offer discount y on product z for life
If cart contains x, suggest product z (full price)
If you want to make this REALLY powerful, give it the ability to adjust the promotion by day. Tomorrow might be "Free Domain Friday", and anybody who buys a VPS gets a free domain. Then it's "SSL Saturday", and anybody who buys a managed VPS or managed dedicated server gets a SSL certificate for only $.99.
This is a feature that's incredibly powerful whether implemented in its most basic form (if cart contains x, and doesn't already contain y, then offer y with a discount) or in a powerful conditional branch (if the customer buys a SSL cert AND the customer already has a VPS AND the current VPS is unmanaged AND it's Saturday THEN (IF the customer has cPanel THEN offer SSL certificate installation for $9.99, else IF the customer has no panel THEN offer SSL installation for $19.99, else IF the customer has Webmin offer SSL installation for $999.99)). I don't believe anyone else has this -- certainly not WHMCS. It would be a huge selling point.