Now it depends on what kind of piano parts you play in your songs. Keyboard players in bands often play background synth and organ parts. This type of playing can be accomplished on 61 key keyboards with unweighted keys. However, singer/songwriters who sing from the piano are usually not playing parts like this. We write our songs on the piano, so the instrument tends to take on a fundamental role in the song. Whether we are performing with just piano and voice, adding background tracks to the mix or have a band to back us the piano/keyboard still is in the forefront. The performance styles of Billy Joel, Sara Bareilles, Michael W. Smith and Mark Schultz all exhibit this (to name a few examples).
Artists like this do not sing and play from a 61 key, unweighted keyboard. They usually perform from a grand piano. When that is not available they play an 88 key, fully weighted, top of the line keyboard. We piano playing singer/songwriters want to feel like we are playing an expressive instrument while we perform.
You see music is all about expression and experience. Musical performers are attempting to express emotions, feelings and ideas through their music. The ideal situation occurs when the performer creates a musical experience that automatically shares these emotions, feelings and ideas with the audience. To do so, we need an instrument at our fingers that does not inhibit our expression.
Guitar players can perform with the very instrument that they originally wrote the song with because it is portable. Every feeling and expression that they experienced while writing the song is still at their fingers while performing. People who write on their piano always have to perform on something else (unless they have the money to ship their piano to each performance). I have never played a keyboard that exactly matches the experience of playing a real piano. Sound resonating from the soundboard of a piano is an experience that speakers cannot replicate. The vibration of the strings traveling through the keys is a sensation that no keyboard replicates. The responsiveness to dynamics and expression found in a great piano cannot be replicated through the midi data expression range of 0 - 120 (although they try to come close).
The only solution is to try to use a keyboard with the best action and sound possible. This solution is often offset by your budget and the strength of your back (plus size of your car). These types of keyboard tend to be more expensive. In addition these instruments tend to be larger and heavier than their less expressive counterparts. Plus, a keyboard will only sound as good as the system that is amplifying it. The connection between the size and weight of a sound system and the quality of sound it produces is a topic for another week (probably next week). However, even with the information already discussed I think you will agree that guitar players have it easier when it comes to being a singer/songwriter.