What are the consequences for NOT having a M-S decoder on the pre-amp or recorder?
It just means that you can't matrix the MS to X-Y to record or monitor the Left-Right signal instead of the MS.
Some recorders have matrices built in which allow the MS to be converted to XY for either recording or monitoring or both. If limited to a single matrix, I find a monitor matrix the most useful option so I can record MS for later processing in post production but hear the matrixed X-Y as a double check that the mics are in the desired place.
Arguably, if you're doiing MS for stereo dialogue/FX booming on a TV/film production, you could possibly do without hearing the stereo version and work on the basis that if the mono sound is right, then the MS stereo will be the same but (variably) wider. I know several TV/film people who work like that but equally I know others who insist that they need to hear the stereo matrix version. I guess there are good arguments on both sides and it comes down to personal preference. For music recording I'd always prefer to go with a monitor matrix, just to be sure.
If there's no monitor matrix in your machine, you could get (at various different price/quality/complexity/flexibility levels) an external box that is just a matrix, or a headphone amp with matrix, or a mic preamp with matrix, or, if you're half competent with a bit of DIY soldering and just want the basics, you can get some seriously good qualitytransformer sets to do MS/X-Y matrixing at mic or line level or, for simple monitoring, Sowter make a nifty little transformer which does a fixed 90 degree MS matrix function, that's specifically designed to sit in a headphone feed. As it's passive, it won't impact on battery life and it's very simple to build into a small box with a headphone socket on one end and perhaps a short flying lead to jack to plug into the headphone out of whatever is recording. Add a bypass switch and you can flip between straight through or matrixed. It's not perfect but is a nice cheap way of getting basic MS monitoring if you need it. I have one of these which lives in my bodgit box of adapters and doobries but for more serious MS work I also have: a Crookwood Paintpot Mk1 preamp which has a built in (fixed ratio) matrix, an AEA MS380TX stereo preamp which has a variable width matrix and some very useful patching and routing options to allow for pretty much any combination of simultaneous MS>X-Y / X-Y>MS allowing recording and/or monitoring in either format and the option of using the matrix in post to go between formats in either direction, an Audio Developments Portaflex AD066/11 stereo preamp/headphone amp (with similar monitoring and routing choices but not at the same quality level as the AEA), and a few digital and analogue mixers with built in MS decoders together with some MS decoder plugins for post work. So lots of choices.