I believe later on the 3rd seat of 11th squad reaches Bankai as well. These are specific skills (mastering one's sword) but being on captain level first requires a level of spirit power (reiatsu), and I think that's something Renji isnt quite up to yet.
He definitely has technique over Ichigo though, he can fight quite evenly with Byakuya on pure "swordsmanship" terms but got owned by the some ability (seems like pure spirit force) from Byakuya that held Renji in place where he got cut up helplessly.
Even before Ichigo reached bankai and became quite a bit more skilled and faster, he achieved the spirit power of a captain first (like on the bridge). No one ever seems to mention (or feel) Renji's paltry spirit force
I feel like he's gonna turn into a Krillin of sorts, weak as hell, but has some sort of "ace in the hole" power (his bankai for example) that lets him be semi-useful, kind of like Krillin and his ability to cut through ANYONE with that buzzsaw bomb.