Absolutely right...
Without a keyboard, u can still write good tunes for sure, but u r gonna miss a lot of fun, for example, on guitar you can't hear the subtle difference between IV9sus4 and IVm11 at most keys, and this kind of little difference could bring more expression and feelings to your music...
There's no better tools than keyboard to experiment and feel harmony, coz it has a key for every note, this means u can play something like LEFT HAND:2-----RIGHT HAND: 34561, which is impossible on guitar. If u are serious of becoming an arranger and not just a player, u should try to familiarize urself with the wonderful instrument.