It's difficult to answer your question about whether it's safe to disable validation without knowing the exact circumstances, but I'd caution against disabling it if you don't do very thorough validation of the path component yourself.
The question mark and ampersand might be dangerous because they are used to pass parameters to an application over an URL. The ampersand is also a problem because html entities can be encoded with it (
ä produces 'ä'). This isn't really a problem in an URL, but if you get your encoding/decoding slightly wrong and insert a get variable that contains these into your page html, you might allow an attacker to include dangerous html tags such as the script tag, iframes etc. The percent character is a problem because it is used to encode special characters in URLs. And of course the < and > are a problem because these are used for HTML tags.
If your application provides a way to accept the request path (or body content) with these characters from just anyone, someone might think of a way to use them to trick your application into doing something unintended.
For example, if you didn't validate the request path and your application inserted the value of the 'title' variable into your page if it was present, this would be harmless when someone requested an url such as
http://www.example.com/profile?user=johndoe&title=John, but someone else might use
http://www.example.com/profile?user=johndoe&title=<script>really bad script here<script> (probably encoding the value of the title using percent url encoding) and have his script run in the context of your page.
It would be even worse if you stored the value of the title variable in a database and later showed it to every user who wanted to look at john doe's profile, because that would run the script whenever anyone looked at john doe's profile. The script might then steal these user's cookies or do some other really horrible and evil thing to them. :-) The example is a bit far-fetched, but the basic problem is not.