While the question has been more than answered there are still some things that seem to me worth adding. The first is that, in my opinion, MatrixForm is "essentially" obsolete. If you wish your matrices always look like matrices (in the output) you can set the format type of output cells to TraditionalForm (use the Appearance tab in the Preferences menu). In fact, you can also set the format type of your input cells to TraditionalForm, although you have to be a little careful if you do that (doing that is not recommended by WRI but it has some well known supporters...).

Alternatively you can use the ConvertTo menu to convert any matrices to TraditionalForm while keeping other cells or expressions in StandardForm (if you prefer that). Finally, if you would like all your matrices always to appear in MatrixForm and avoid these evaluation problems, you can evaluate at the beginning of your Mathematica session

$Post = If[MatrixQ[#], MatrixForm[#], #] & 

or you can put it into an init file and have it evaluate automatically. (Of course you can use TraditionalForm in place of MatrixForm).

Coming back to the issue of TraditionalForm vs MatrixForm for matrices: the only problem I can see with using the former is that it looks "too nice" so that if the rest of your output is in StandardForm the style of your matrices will not match the rest of your output. But other than that I can't think of any use for MatrixForm.

While the question has been more than answered there are still some things that seem to me worth adding. The first is that, in my opinion, MatrixForm is "essentially" obsolete. If you wish your matrices always look like matrices (in the output) you can set the format type of output cells to TraditionalForm (use the Appearance tab in the Preferences menu). In fact, you can also set the format type of your input cells to TraditionalForm, although you have to be a little careful if you do that (doing that is not recommended by WRI but it has some well known supporters...).

Alternatively you can use the ConvertTo menu to convert any matrices to TraditionalForm while keeping other cells or expressions in StandardForm (if you prefer that). The keyboard shortcut for this is Ctrl-Shift-T. Finally, if you would like all your matrices always to appear in MatrixForm and avoid these evaluation problems, you can evaluate at the beginning of your Mathematica session

$Post = If[MatrixQ[#], MatrixForm[#], #] & 

or you can put it into an init file and have it evaluate automatically. (Of course you can use TraditionalForm in place of MatrixForm).

Coming back to the issue of TraditionalForm vs MatrixForm for matrices: the only problem I can see with using the former is that it looks "too nice" so that if the rest of your output is in StandardForm the style of your matrices will not match the rest of your output. But other than that I can't think of any use for MatrixForm.

While the question has been more than answered there are still some things that seem to me worth adding. The first is that, in my opinion, MatrixForm is "essentially" obsolete. If you wish your matrices always look like matrices (in the output) you can set the format type of output cells to Traditional form (use the Appearance tab in the Preferences menu). In fact, you can also set the format type of your input cells to TraditionalForm, although you have to be a little careful if you do that (doing that is not recommended by WRI but it has some well known supporters...). Alternatively you can use the ConvertTo menu to convert any matrices to Traditional form while keeping other cells or expressions in Standard form (if you prefer that). Finally, if you would like all your matrices always to appear in MatrixForm and avoid these evaluation problems, you can evaluate at the beginning of your Mathematica session $Post = If[MatrixQ[#], MatrixForm[#], #] & or you can put it into an init file and have it evaluate automatically. (Of course you can use TradtionalForm in place of MatrixForm). Coming back to the issue of TraditionalForm vs MatrixForm for matrices: the only problem I can see with using the former is that it looks "too nice" so that if the rest of your output is in StandardForm the style of your matrices will not match the rest of your output. But other than that I can't think of any use for MatrixForm.

While the question has been more than answered there are still some things that seem to me worth adding. The first is that, in my opinion, MatrixForm is "essentially" obsolete. If you wish your matrices always look like matrices (in the output) you can set the format type of output cells to TraditionalForm (use the Appearance tab in the Preferences menu). In fact, you can also set the format type of your input cells to TraditionalForm, although you have to be a little careful if you do that (doing that is not recommended by WRI but it has some well known supporters...). 

Alternatively you can use the ConvertTo menu to convert any matrices to TraditionalForm while keeping other cells or expressions in StandardForm (if you prefer that). Finally, if you would like all your matrices always to appear in MatrixForm and avoid these evaluation problems, you can evaluate at the beginning of your Mathematica session

$Post = If[MatrixQ[#], MatrixForm[#], #] & 

or you can put it into an init file and have it evaluate automatically. (Of course you can use TraditionalForm in place of MatrixForm).

Coming back to the issue of TraditionalForm vs MatrixForm for matrices: the only problem I can see with using the former is that it looks "too nice" so that if the rest of your output is in StandardForm the style of your matrices will not match the rest of your output. But other than that I can't think of any use for MatrixForm.