I've never heard "argu r able", and I would not describe it as an "accepted alternative pronunciation".
It seems fairly plausible to me that some people might use this pronunciation, because there is an /r/, or an R-colored vowel, earlier in the word. Words with multiple R sounds are somewhat prone to losing one (via "dissimilation"), and somewhat less commonly, words with a single R sound may have variant pronunciations with more than one due to "assimilation" (as with sherbet, which has a variant pronunciation "sherbert"). See my answer to 'Forward' pronounced more often as 'foward'? for some more information about this phenomenon.
Variation between /ju/ and /jər/ before a vowel is known to occur in at least one other word, barbiturate (which has the stigmatized variant barbituate, as mentioned in a comment by Janus Bahs Jacquet).