Can DAQ devices be used as Oscilloscopes?
DAQ systems can make very functional low-speed oscilloscopes, with a number of caveats:


You're not going to get a very broad voltage range. Most of them will maybe do ±10V input range.
Probably won't support offset subtraction on the inputs, like scopes do. 
DC-coupled only, unless you supply the series cap.
Inputs can be low(ish)-impedance (some may have buffer amps, on cheap ones the input may literally just connect to the ADC pin). Not the 1MΩ standard that scopes have.
Most importantly:


PC based oscilloscope interfaces suck

Also, it's likely a DAQ won't even have a traditional oscilloscope-like software tool. You may have to write your own.


Anyways, if you have a situation where you have fixed or low voltages, and don't mind doing a bunch of work on the PC end, a DAQ could be used as rather pokey oscilloscope. 

They're really different tools, though, and while they do share some characteristics, they have very different intended uses, and this tends to show in their approach and the software design considerations.



It's also worth noting that most DAQ systems are designed for continuous, rather then triggered data acquisition. This means you're maximum sample rate is largely limited by the interface the DAQ uses.  

For example, USB2 only has 480 Mbps(more like 400 Mbps real-world) bandwidth. As such the best sample rate that could ever be achieved would be 50 Msps(million samples per second) at 8 bits resolution, and very few implementations will even approach that. Somewhere in the range of 1-10 Msps at 8 or 16 bits is more realistic. Extracting all the available bandwidth from USB is very challenging.

Another consideration is what you're going to do with all the data. 1 Msps is a lot of data. If the 1 Msps stream is 16 bits, that's 2 Megabytes of data per second, or a gigabyte every 8 minutes. I don't know what you're intending to do with this pseudo-oscilloscope, but you can't just take samples willy-nilly, unless you're just displaying them and then immediately discarding them.



I've actually written a minimal real-time visualization tool for some IOtech-branded DAQ systems at work. It's kind of an oscilloscope. It works well, but I've also designed all the PCBs that interface with the DAQ system, so I could design them to work to the DAQ system's input specifications.