She doesn't spam her abilities so she can't stack it well. If you spam your Q, you won't have hard CC when you need it. If you spam your W, you'll go OOM really fast and it'll hurt your ADC not to have a heal when you need it. You can "spam" E since it won't, comparatively, hurt you in any way.. but it's got an 11 second CD (and the other 2 spells have similarly long cooldowns) so you can't even use it that much.

Compare this to champions who use Tear of the Goddess. Cassiopeia's E has a 0.75 second CD, Q has a 3.5 second CD. Ezreal's Q has a 5.5-4.5 second CD. All 3 of these abilities can be spammed because they have low CDs, are their main damage sources, are useful (for poke (unlike Nami's Q, W or E being spammed)), don't hurt their lane partner (Nami's abilities are necessary for engage, disengage and peel) AND they both benefit greatly from Tear's upgrades. Seraph's Embrace is not necessary for Nami at all and it's way too expensive for a support item. 3200 gold PLUS you have to stack Tear, versus her usual items Redeption 2100, Ardent 2300 and Athene's 2250 which all have good prices for a support champion and of course are all also helpful for Nami's team (wheras Seraph isn't).