Going by this:
http://wiki.bluegartr.com/bg/Magic_Evasion
Rune effect merits with 3 of the same rune up gives between a 15% and 30% magic evasion rate (likely closer to 30% as I'm fairly sure we can push > 50% base resist rate without merits) and it can be full timed.
If you are fighting something and your mag eva is capped without the merits (Likely something insignificant in that case) then even a +20% resist rate bonus on pflug will only be reducing the number of received enfeebles by 1 out of 100 casts (assuming pflug is applied in a diff step, otherwise it's also doing nothing).
Especially true of light/dark based enfeebles as we are less likely to be hitting capped mag eva for these elements with the lack of barlight/dark.
It's extremely rare to have an enfeeble land without pflug up using 3 of the same rune. I could maybe see it being more useful if you were using 3 diff runes & dropping your base evasion below a 50% resist rate. However, it's still 5% evasion per rune minimum purely from magic evasion, it's applied to damaging spells too and it's up full time.